PRINCIPLES OF COMPARATIVE PHYSIOLOGY ** 6 BY THE SAME AUTHOR PRINCIPLES OF HUMAN PHYSIOLOGY Fotjkth Edition. 8vo. 28s. A MANUAL OF PHYSIOLOGY. Second Edition. Fcap. 8vo. 12s. 6d. PRINCIPLES \7.l OF COMPARATIVE PHYSIOLOGY. BY WILLIAM B. CABPENTEB, M.D., F.B.S., F.G.S. EXAMINER IN PHYSIOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF LONDON; PROFESSOR OF MEDICAL JURISPRUDENCE IN UNIVERSITY COLLEGE; PRESIDENT OF THE MICROSCOPICAL SOCIETY OF LONDON, ETC. ETC. xfy ffijjm flwitofo ffl&aab ©ngrabhrgs. JFourti) tuition. LONDON: JOHN CHURCHILL, NEW BURLINGTON STREET V {Established in Princes Street, Soho, 1784.) M DCCC LIV. > Cartibridc* fcfifty .roiwy* M*^ * V ■ ■ «$fcft itfany > TO SIR JOHN F. W. HERSCHEL K.H. F.R.8. L. AND E. ETC. THIS VOLUME IS MOST RESPECTFULLY DEDICATED, AS A TRIBUTE DUE ALIKE TO HIS HIGH SCIENTIFIC ATTAINMENTS, AND MORAL WORTH, AND AS AN EXPRESSION OF GRATITUDE FOR THE BENEFIT DERIVED FROM HIS u DISCOURSE ON THE STUDY OF NATURAL PHILOSOPHY," BY THE AUTHOR. ^™ * ■ ^ PREFACE. << SCIENCE IS THE KNOWLEDGE OF MANY, ORDERLY AND METHODICALLY DIGESTED AND ARRANGED, SO AS TO BECOME ATTAINABLE BY ONE." Sir John F. W. Herschel. The success of the Third Edition of the " Principles of Physiology, General and Comparative/' — as evinced, not merely by its rapid sale, but by the numerous expressions of high appreciation which it drew forth from those most competent to j udge of its merits, — has encouraged the Author to carry into effect a change of plan which had suggested itself to him during its preparation. For having been led-on by the desire of rendering his work as complete as possible, to enlarge it to the utmost admissible dimensions of a single volume, he felt that it would be impossible to do justice to any subsequent extensions which its subject might receive, without making some alteration in its form. And this conclusion acquired a greatly-increased force, when the demand for a new Edition led him to survey the deficiencies, which, notwithstanding all his care, had been left in the former one ; and to estimate the amount of new matter, not only deserving but requiring notice, which the diligence of observers in various departments of this comprehensive Science had accumulated in the short interval. Instead of dividing the entire Treatise into two Volumes, however, as suggested to him by many of his friends, the Author has preferred to divide its subjects, and to treat of them separately though connectedly. The present Volume, therefore, consists of the " Comparative Phy- siology" of the last Edition, extended from 530 pages to 744, and with 300 illustrations instead of 130. The First Chapter (the Eighth of the former Edition) has been entirely re-written, with the view of bringing into greater prominence the general doctrine of Progress from the General to the Special; the enunciation of which by Von Baer (nearly thirty years ago) appears to the Author to mark a most important era in the Philosophy of Physiology, although it is far from having received the notice which it deserved from various subsequent writers who have ♦ * • Vlll PREFACE. followed in the same track. The principal additions and alterations in the succeeding Chapters (as to the general arrangement of which no '*y) which relates to the Water System, and in the part of has Chapter XL which treats of the Sexuality of the Cryptogamia; in regard to which latter point the Author would observe, that the course of recent discovery has fully borne-out the anticipations he expressed in the former edition. The whole work, however, has been most care- fully revised; and the Author ventures to think that the present Edition more completely represents the state of the Science at the period of its publication, than any of its predecessors have done. He can honestly say that he has spared no time or labour in its preparation, which it has been in his power to bestow. And he looks with hope, therefore, to a continuance of that friendly indulgence with regard to errors and short- comings, which has been so liberally afforded on previous occasions. As to certain points on which his opinions have undergone modification, he can again refer with satisfaction to the following passage in the Preface to his former editions: — "Truth is his only object; and, even if his own doctrines should be overthrown by more extended researches, he will rej oice in their demolition, as he would in that of any other error. The character of the true philosopher as described by Schiller, always loved truth better than his system, — will ever, he trusts, be the goal of his intellectual ambition." In attempting to embody in a Systematic Treatise the general aspect of Physiology or any other Science of like comprehensiveness, it will be obvious that an Author, however extensive his own range of acquire- ment, must largely avail himself of the labours of others ; and that the scientific character of such a treatise must depend, not so much on the amount of original matter it may contain, as on the degree in which * ■ " the knowledge of many" has been " orderly and methodically digested and arranged, so as to become attainable by one." It is by this standard that the Author desires his work to be tried; and he cheerfully leaves the verdict to -the judgment of those, who are qualified by their own knowledge of the subject to pronounce it. He feels it due to himself, however, to state that he has devoted considerable time and attention to the verification of the statements of other observers, especially on points under dispute, — a kind of labour which is but little appreciated by those, who contemptuously designate works like the present as "mere compilations;" and that a large amount of materials, drawn from his PREFACE. wnZ&A q TT soattered throagb the voA - » ™ M »- been T d ! T i g ^ lMt ^ «"*' I"-*—* lad he ~ ptrr ; *, his °° nstant aim has ^ *> ™*-* «. one ^oToftT ^ ^ meth ° di0aUy ' mthCT than *° *»» any comi„„ ri T U l IOnS *° PM »***■«»* rather than te he read 7 L w fT g , ^^ "^ ** the attent ^ <* his shown 1 the d ' mWe ° Ter ' ** "***** m ^ "e as much observed b v tt Pm ^ "JT ^^ "*"" ** «* I*™ he h a/f_? r ,??" Comtoed - d arranged his materials, to staJmlTthiT ^ *° ImPart a MW "* me ^eted valu statements, which, in their previously isolated condition were of comparatively insignificant import Procure upon each department of the sutW i+ : , , r ted that he shouid he e^y ^ ,^J J£ ^ le s^: » V ; " Parti ° Ular ^rtmenJinto detli, ^ or evenas iTwl^^ ^ «™ "*«* " ^eieneies, be estimated W s. , mUSt be « that his work may -bat 72 1 T r ^ '' "* ratb6r by VLat * *«* than * by me e tmon f ^ W ° UM haTC ^ &r eMier *» expand it fo»d o 2Zt2 , ^ PreSen ' dimenSi ° M ' * fl an it has heen »ow oeenpST accumulated maaa within the space which it even "S^ZS t"? r ' S endeaVO "' Where ™ r Praa - bk > to draw the Treaties ' £f* " *? ^ ** * m — *«* *•" *» original freqnenl !rf, "T*"**- « and thns to avoid tke errors which too however toT- ">»« transmission. To have attempted, wever, to assign eaeh individual fact to its original disco™. 7 doctrine to its first ennnciator, wonld have augmented t b, W ", vehime tar beyond the dimension, appropriat! "-Book"' £ Part ' *„ «, * compelled to lnmt his references, for the most Pai t, to those new facts and doctrine* ,,,1, • i, , , become part of the common stock "^ * ** " U *° ^ o> such references has W , * , Ph ^ological Science. The nnmher tbe - Xndex of Aut W h it" T^ " ^ I *~* ««*» i and Authors which has been added, will, it i s hoped, be ound X PBEFACE. •eadiness which useful in enabling the reader at once to turn to the notice of any original observation that he may desire to retrace. The Illustrations not his own which likewise have received numerous important additions, are referred to their originals in the list at the commencement of the Volume ; and this list will also afford useful assistance to those, who may desire to carry-out their enquiries in any particular direction. The Author cannot bring his task to a conclusion, without expressing the great obligations under which he lies to his friend Mr. T. H. Huxley, not only for many valuable suggestions, but also for the he has on all occasions evinced, to impart to him whatever he might seek from his own extensive stores of original and acquired information ; nor without paying his tribute of regard to the memory of his lamented friend Mr. G. Newport, whose premature death has deprived British Science of one of its most ardent and disinterested votaries, at a time when he was beginning to reap, in the appreciation of his discoveries on the Impregnation of the Amphibia* the credit so justly due to his laborious, accurate, and sagacious researches, in the new field to the cultivation of which he had latterly applied himself. It is the Author's intention to reproduce the « General Physiology" of his former Edition, as a companion-volume to the present, so soon as the numerous demands upon his time may permit him to bestow upon that part of his revision the careful attention which it requires. University Hali-, London, June 1, 1854. * In a Postscript to the work referred-to in the note to p. 536 written almost contem- poraneously with Mr. Newport's decease, Prof. Bischoff states that he has himself con- firmed Mr N.'s observation of the penetration of the Spermatozoon into the ovum of the Frog, and gives him full credit for the determination of this important fact. MMV ^^■B '-K^^L^^^H if*fll TABLE OF CONTENTS i. 2. 3. 4. 5. 6. i . CHAPTER I. ON THE GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. Analysis and Comparison of Phenomena afforded by Organic Structure : Homology and Analogy .... Conformity of Structure of each group to General Design or Archetype :■ Egress from General to Special in its various modifications . Diversities in Grade of Development . General Survey of Vegetable Kingdom. Protopkyta . Thallogens (Algae, Lichens, Fungi) A crogens (Hepaticae, Mosses, Ferns) . . Phanerogamia ...... General Survey of Animal Kingdom. Protozoa (Porifera, Rhizopoda, Infusoria) .... Radiata (Polypifera, Acalephas, Echinodermata) . Mollusca (Bryozoa, Tunicata, Brachiopoda, Lamellibranchiata, Gasteropoda, Pteropoda, Cephalopoda) .... A rticulata (Entozoa, Annelida, Myriapoda, Insects, Crustacea,' Arachnida) Vertebrata (Fishes, Reptiles, Birds, Mammals) . Progress from General to Special in Development Rudimentary Organs ........ Monstrosities ......... Geological Succession of Organic Life PAGE 1 10 17 22 23 28 33 39 41 50 59 71 95 101 105 107 CHAPTER II. 1. GENERAL VIEW OP THE VITAL OPERATIONS OP LIVING BEINGS, AND OP THEIR MUTUAL RELATIONS. Analysis and Classification of Phenomena presented by Vital Action 2. Mutual Relations of Organic and Animal Functions 3. Organic Functions separately considered 4. Animal Functions separately considered 5. Progress from General to Special in Function 121 123 125 128 129 • * XII CONTENTS. CHAPTER IIL OF ALIMENT, ITS INGESTION, ANI) PREPARATION in Plants 1 . Sources of Demand for Aliment . 2. Nature of the Alimentary Materials 3. Ingestion and Preparation of Aliment 4. Ingestion and Preparation of Aliment in Animals Agastric Animals Unicellular Animals . Polystome Animals . Oral Apparatus Prehensile Appendages Reducing Apparatus Digestive Apparatus . Nature of Digestive Process CHAPTER IV. OF ABSORPTION AND IMBIBITION PAGE 132 139 151 153 153 153 157 158 162 164 169 183 1. General Considerations 2. Absorption in Vegetables 3. Absorption in Animals 186 193 199 CHAPTER V. NUTRITIVE 1. General Considerations 2. Circulation in Vegetables . 3. Circulation in Animals Absence of Special Circulation in Certain Classes Circulation in Eadiata : — Echinodermata Circulation in A rticulata : — Annelida . Myriapoda Insects Arachnida Crustacea . Circulation in Mollusca : — Bryozoa Tunicata . Brachiopoda Lamellibranchiata Gasteropoda Cephalopoda Circulation in Vertebrata : — Fishes Eeptiles Birds Mammals Forces which move the Blood Development of Circulating Apparatus Malformations of Circulating Apparatus 212 213 221 225 227 229 235 237 239 242 245 246 249 250 251 253 255 258 263 263 264 268 277 CONTENTS. * • * Xlll 1. 3. General Considerations Respiration in Plants Respiration in Animals Aquatic Respiration CHAPTER VI. OF RESPIRATION. Water- Vascular System :-Rotifera Protozoa, Zoophyt Echinodermata es, and Branchial Respiration Atmospheric Respirati Entozoa Bryozoa Tunicata Brachiopoda . Lamellibranchiat Gasteropoda . Cephalopoda . Annelida Crustacea Pishes . Batrachia a ion :— Myriapoda Insects Arachnida Pishes (air-bladder) Perennibranchiata Reptiles Birds Mammal n mammals development of Respiratory Apparatus . Aiteratinna ^w^^^a 1 t> . . . effected Acaleph se CHAPTER VII. l. 2. General Considerations Exhalation in Plants 3. Exhalation in Animals OF THE EXHALATION OE AQUEOUS VAPOUR 1 General Considerations CHAPTER VIII OF NUTRITION. Term of Duration of Individual Parts Assimilation and Formation . 2. Nutrition in Vegetables Growth and Multiplication of Cells Assimilating Process in Vascnlar Plants Production of Vegetable Organic Compound PAGE 280 283 290 294 295 297 298 300 301 304 305 306 307 308 310 312 316 317 318 322 323 325 325 327 330 332 333 339 339 346 351 352 356 360 360 369 372 XIV CONTENTS. 3. Nutrition in Animals Assimilation of Nutritive Materials Chyle and Lymph Vascular Glands Composition and Properties of Blood of Vertebrata Nutritive Fluid of Zoophytes Echinodermata Articulata Mollusca . Growth and Multiplication of Cells Production of Animal Organic Compounds Conditions of Nutritive Activity in Animals PAGE 379 379 381 384 386 392 392 393 395 396 401 405 CHAPTER IX. OF SECRETION AND EXCRETION 1 . General Considerations . 2. Secretion in Vegetables . • • 3 Secretion in Animals • Structure of Glands in general . The Liver, and the Secretion of Bile . Biliary Apparatus of Invertebrata Vertebrata . Development of Liver Properties and uses of Bile Of the Kidneys and the Urinary Excretion Urinary Apparatus of Invertebrata Vertebrata Development of Kidney Composition and Properties of Urine Cutaneous and Intestinal Secretions Special Secretions . Metastasis of Secretion 407 409 410 411 417 417 421 424 425 429 429 429 432 434 438 438 439 CHAPTER X. EVOLUTION OF LIGHT, HEAT, AND ELECTRICITY 1. General Considerations . 2. Evolution of Light . • • Evolution of Light in Vegetables Evolution of Light in Animals Luminosity of the Sea Luminous Insects 441 442 442 443 443 446 CONTENTS, XV 3. Evolution of Heat . Evolution of Heat in Vegetables Evolution of Heat in Animals Cold-blooded Animals Insects Warm-blooded Animals Conditions of Evolution of Heat 4. Evolution of Electricity . Evolution of Electricity in Vegetables Evolution of Electricity in Animals Electric Fishes . PAGE 449 450 452 452 454 457 460 461 462 463 467 CHAPTER XI. 2 OF GENERATION AND DEVELOPMENT. 1. General Considerations .... Developmental and Regenerating Power Multiplication by Gemmation True Generative Process Alternation (so-called) of Generations Generation and Development in Plants Multiplication of Phytoids . Generation and Development of Protophyta Alg33 . Characese Lichens Fungi . Hepaticse and Ferns . Equisetaceaa Lycopodiaceas Marsileaceae. Gymnospermese n . Angiospermous Phanerogamia feneration and Development in Animal Mosses 3. ,s Multiplication of Zooids Development and Actions of Spermatozoa Development and Structure of Ova Fecundation of Ova, and subsequent Changes Generation and Development of Protozoa Infusoria Porifera Generation and Development of Radiate Polypif era Compound Hydroida Acalephaa Echinodermata 473 476 480 481 482 483 485 486 491 495 497 499 502 505 511 512 513 514 515 528 528 529 534 535 539 539 543 544 544 549 553 561 XVI CONTENTS, Generation and Development of Mollusca Bryozoa Tunicata Brachiopoda Lamellibranchiata Gasteropoda Cephalopoda Generation and Development of Articulata Entozoa Rotifera Annelida Myriapoda Insects . Crustacea Cirrhipeda Arachnida Generative Apparatus of Vertebrata Fishes . Reptiles Birds . Mammals Embryonic Development of Vertebrata Area Germinativa . Formation of Amnion Development of Allantois Formation of Placenta Conditions determining Sex . Lactation : — Composition of Milk 4. On the Laws of the Exercise of the Reproductive Species and Varieties . . . Hybridity ...... Modifying influence of External Conditions Origination of New Varieties Transmission of Acquired Peculiarities . Function PAGE 568 568 570 574 574 576 580 585 585 590 591 595 596 602 605 607 609 610 611 612 615 619 620 622 625 626 629 630 632 632 634 634 637 638 CHAPTER XII. OF THE SENSIBLE MOTIONS OF LIVING BEINGS 1. General Considerations 2. Motions of Plants 640 642 3. Motions of Animals . . . . . . . . . .645 CHAPTER XIII. OF THE FUNCTIONS OF THE NERVOUS SYSTEM ft 1. General Considerations . . . '. . 2. Comparative View of the Nervous System in the Animal Series No Evidence of Nervous System in Zoophytes 648 651 651 CONTENTS. • • XV11 Nervous System of Radiata . AcalepL.se Echinodermata Nervous System of Mollusca Bryozoa . Tunicata Brachiopoda Lamellibranchiata Gasteropoda . Cephalopoda . Nervous System of Articulata Entozoa . Annelida Myriapoda Insects . Crustacea Arachnida Nervous System of Vertebrata Fishes . Reptiles . Birds Mammalia History of Development of Brain Functions of the Cranio-Spinal A Spinal Cord Medulla Oblongata Sensory Ganglia . Functions of the Cerebellum Functions of the Cerebrum xis General Summary Sympathetic PAGE . 653 . 653 . 653 . 654 . 655 • 655 . 656 . 656 . 658 . 660 . 663 . 670 . 670 . 670 . 670 . 672 . 673 . 675 . 677 . 681 . 681 . 682 679—684 . 685 . 687 . 689 . 690 . 696 • 698 . 703 • 704 CHAPTER XIV. OP SENSATION AND THE ORGANS 1. Of Sensation in General 2. Of the Sense of Touch, and its Instruments ' *• Ut the Sense of Taste, and its Instruments . *- Of the Sense of Smell, and its Instruments *• Of the Sense of Hearing, and its Instruments 6. Of the Sense of Sight, and its Instruments OF THE SENSES 709 712 715 716 718 725 «4 CHAPTER XV. O* THE PRODUCTION O* S0TJNI)S BY ANMALg 738 b LIST OF ILLUSTRATIONS. PIG. 1. Pterodactylus crassirostris (xxvi.) 2. Different forms of Anterior Member . 3. Diagram illustrating the Nature of Limbs (lxiv.) 4. Group of Anatifa Icevis (xxiii.) . 5. Anatomy of Anatifa Icevis (xxiii.) 6. Development of Balanus balanoides (vn.) 7. Comparison of Leucifer with. Lepas (xxv.) 8. Ophiura texturata . 9. P entacrinites briareus (xvi.) 10. Hermospora transversalis (lxix.) 11. velutina 12. Mesogloia vermicularis (lxix.) . 13. Z onaria plantaginea (lxix.) 14. Dasya huetzingiana (lxix.) 15. Marginaria gigas (lxix.) . 16. Parmelia acetabulum (lxix.) 17. Sphcerophora coralloides (lxix.) 18. Stysanus caput-medusce (lxix.) 19. Clavaria crispula (lxix.) . 20. jEcidium tussilaginis (lxix.) 21. Marchantia polymorpha (lxix.) 22. Fissidens bryoides (lxix.) . 23. Marchantia polymorpha with antheridia (lxix.) 24. Marchantia polymorpha with pistillidia (lxix.) 25. Polytrichum commune (lxix.) . 26. Trichomanes speciosum (lxix.) . 27. Frond of Scolopendrium vulgar e (lxix. ) 28. Frond of Osmunda regalis (lxix.) 29. Equisetum arvense (lxix.) . 30. Lycopodium cernuum (lxix.) . 31. Marsilea quadrifolia (lxix.) 32. Ideal Plant (lxxviii.) 33. Amoeba princeps (xxxv .) . 34. Hydra fusca (xc.) . . - 35. Diagrammatic section of Actinia (lxxx.) 36. Structure of Cyancea aurita (xxxvi.) 37. Anatomy of Asterias aurantiaca (lxxxviii.) 38. Comatula rosacea (xxxix.) PAGE 4 5 9 12 12 14 14 19 19 22 22 24 24 24 24 25 25 26 27 27 28 28 29 29 29 30 30 30 30 31 32 33 40 43 44 46 47 48 LIST OF ILLUSTRATIONS. XIX FIG. 39 40 41 42 43 44 45. 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77. 78 79 80 81 82 83 84 85 86 87. Echinus mammillatus (xxiii. ) Anatomy of Holothuria tubulosa (xxni.) Ohitonellus and Chiton (xvni.) Salpa maxima (xxm.^ Anatomy of Mactra (xxxiv.) Anatomy of Paludina vivipara (xxn.) Shells of Gasteropod MollusJcs (xvni.) &yalcea, Criseis, and Clio (xvni.) Sepia officinalis (xvni.) Embryoes of Nudibranchiate Gasteropoda (n.) Laguncula repens (xcv.) Anatomy of Aplysia (xxn. ) Botryllus violaceus (xxiii.) Anatomy of Strongylus gig as (xi.) Nephthys Hombergii (xxiii.) lulus (xxxiv.) Scolopendra (xxxiv.) Section of the trunk of Melolontha vulgaris (lxxxiv.) Ideal Section of Sphinx ligustri (lxiii.) Anatomy of Cancer pagur us (xxxiv.) Inferior surface of Limulus moluccanus (xxxiv.) Cyclops quadricornis (vi.) Diagram of Archetype Vertebral Skeleton (lxv.) Ideal Section of a Mammal (lxiv.) Oblique view of Vertebra of Cod (lxxiv.) Bimanus and Seps (xxxiv.) Emysaura Serpentina (xxvu.) Skeleton of Ichthyosaurus (xxvi.) Skeleton of Plesiosaurus (xxvi. ) Skull of M ososaurus (xxi.) Portion of jaw of Megalosaurus (xxi.) Portion of lower jaw and teeth of Iguanodon (lvh.) Lower jaw of Phascolotherium Bucklandii (lxvii ) Molar tooth of A siatic Elephant (xxi. ) Duplicative subdivision of cells of Chlamydomonas (xxxv ) ^arly stage of Mammalian Ovum (xix.) and Young of Volvox Micothoe astaci (xcvi.) Ogygia Buchii (xvn.) and Limulus moluccanus (xxxiv.) Metamorphosis of Carcinus mamas (xx. ) . Homocercal and Heterocercal Tails of Fish Skeleton of Palwotherium magnum (xxi.) Molar tooth of Mastodon (xxi.) Caryocrinites ornatus (xv.) Lingula anatina (xvni.) Section of shell of Nautilus pompiMm (xvm ) Exterior view and section of Orthoceratite (xvni. Cephalaspis Lyellii , V Skull of Labyrinthodon Skull of Myncosaurus (lxviii. ) 62 PAGE . 48 . 49 . 51 . 51 . . 52 . 53 . 53 . 54 . 54 . 54 . 55 . 58 . 60 . 61 . 63 . 63 . 65 . 68 . 69 . 70 . 70 . 73 . 75 . 78 . 80 . 81 . 82 . 82 . 84 . 85 . 8b . 94 . 94 . 95 . 95 . 100 . 108 . 108 . 109 . 110 . Ill . 112 . 113 . 113 11 3 114 115 115 LIST OF ILLUSTRATIONS. Skeleton of Mylodon (lxvi.) mag FIG. 88. 89. Pitchers of Dischidia Rafflesiana (c.) 90. Polygastric Animalcules, according to Ehrenberg (xxxv.) 91. Section of young branch of Alcyonium stellatum (xxxiii.) 92. Digestive apparatus of Rhiz ostoma (xxiii.) 93. Digestive apparatus of Thaumantias (xxxviii.) 94. Holoihuria phantapus (xxiii.) 95. Anatomy of Echinus lividus (xxiii.) 96. Rotifer vulgaris (xxxv.) . .... 97. Compound stomach of Sheep .... 98. Section of part of the Stomach of Sheep (xxxvu.) 99. Portions of Campanularia gelatinosa, natural size, and 100. Sections of Alcyonian Polype (xxxiii.) . 101. Structure of Polycelis kevigatus (lxxi.) . 102. Cydippe pileus (xl.) and Beroe Forskalii (xxiii.) 103. Digestive apparatus of Annelida (xxiii.) 104. Eolis Inca, sl Nudibranchiate Gasteropod (n.) . 105. Digestive apparatus of Ammothea pycnogonoides (lxx 106. Digestive apparatus of My gale (xxiii.) 107. Digestive apparatus of Common Fowl (xxxiv.) 108. Villi of Human Intestine 109. Longitudinal Section of Stem of Italian Reed (lxxviii.) 110. Laticiferous Vessels (lxxxi.) .... 111. Blood-vessels of Frog's foot (xcix.) 112. Formation of Capillaries in Tail of Tadpole (l.) 113. Circulating apparatus of Terebella conchilega (xxx.) 114. Circulating apparatus of Eunice sanguinea (xxx.) 115. Circulating apparatus of Arenicola ^iscatorum (xxx 116. Dorsal vessel of Scolopendra (lxiii.) 117. Circulating system of Scolopendra (lxiii.) 118. Circulating system of B uthus (lxiii.) 119. Heart of M ygale (xxiii.) .... 120. Circulating system of Lobster (xxviii.) . 121. Anatomy of A maroucium proliferum (xxix.) . 122. Circulating system of Salpa maxima (xxiii.) . 123. Circulating system of Pinna marina (xxxn.) . 124. Circulating system of Snail (xxxiv.) . . 125. Circulating system of Octopus (xxiii.) . 126. Circulating system of J^M (xxiii.) . 127. Anatomy of A mphioxus (lxiv.) 128. Circulating system of Lizard (xxxiv.) 129. Respiratory Circulation in Tadpole (xxxiv.) 130. The same in transition state (xxxiv.) 131. The same in the perfect Frog (xxxiv.) 132. Diagram of the Circulation in Birds and Mammals 133. Vascular area of Fowl's egg (xcix.) 134. Diagram of formation of great Arterial trunks in Fowl 135. Diagram of Circulation in Human Embryo (en.) 136. Water-vascular system of Tcenia solium (xi.) . nified ) ) ('xxxiv.) ( PAGE . 116 . 152 . 156 . 157 . 158 . 160 . 161 . 165 . 166 . 168 . 168 xciv.) 170 . 172 . 173 . 174 . 175 . 176 . 177 . 177 . 179 . 201 . 214 . 218 . 223 . 224 . 232 . 232 . 234 . 236 . 236 . 240 . 242 . 243 . 246 . 248 . 250 . 252 . 254 . 255 . 257 . 259 . 260 . 260 . 261 . 263 . 269 . 271 . 275 . 298 LIST OF ILLUSTRATIONS. FIG. 137. Anatomy of Fasciola hepaticum (xi. ) 138. Anatomy of Perophora (lvi. ) . . ] 139. Portion of Branchial sac of Perophora (lvi.) .' 140. Respiratory apparatus of Pholas crispata {in.) 141. Branchial lamina of Pholas crispata (m.) 142. Portion of gill of Doris Johnstoni (n.) . 43. D or i s Johnstoni, showing tuft of external gills (n.) 144. Cephalic tuft of Sahella unispira (xxxiv.) 145. Branchial arch and leaflets of Fish . 146. Lamprey, showing branchial orifices 47. Proteus anguineus, showing external branchiae (xxvu.) 148. Axolotl, showing external branchiae (xxvu.) 149. Tracheal system of Nepa cinerea (xxxiv. ) 150. Lepidosiren paradoxa (xxxiv. ) 151. Respiratory organs of Prog (xxxvu.) 152. Lungs oiBimanus, Bipes, and Coluber (xxxvu.) 153. Section of Lung of Turtle (xii.) 153*. Pulmonary apparatus of Pigeon (xxxvu. ) 154. Capillaries of air-cells of Human Lung 155. Vertical Section of leaf of Mium album (xiv.) 1 56. Under surface of leaf of Mium album (xiv. ) 157. Surface and Section of frond of Marchantia polymorpha (lviii 158. Sudoriparous Gland from Human Hand (xoix.) ) 160* p!!^l 0f , L !r f . O f/r W :. Sh0willg P rimordial brides of cells (xlh Portions of Nitella fiexilis, natural size and enlarged (lxxxi ) i fi9 ^rculation of fluid in hairs of Tradescantia Virginica (lxxxi. 1 «q* XT™ 118 Stag6S ° f devel °P ment of Seematococcus binalis (xliii ) 1 (u r7 ° CeSS ° f ^-^^Plication in Conferva glomerata (lix. ) 04. development of zoospores of Achlya prolifera (xcm.) 1«« ^P 11 ^ 011 of Cartilage-cells by subdivision (liii.) I Ob. Section of branchial Cartilage of Tadpole (lxxix.) ^07. Endogenous cell-growth, in cells of Meliceritous Tumour (xli ) tells with radiating fibres (i.) Capillary network around follicles of Parotid Glan Glandular follicles of Stomach (lxi.) Mammary Gland of Ornithorhyncus (lxi.) 172. Rudimentary Pancreas, from Cod (lxi.) . 173. Lobule of Parotid Gland of Human Infant (xoix.) 174. Biliary tubuli of Musca carnaria (lii.) . 175. Hepatic caecum of Astacus affinis (lxi.) . 176. Lobules of Liver of Squitta (lxi. ) . 177. Glandular cells of Human Liver 168. 169. 170. 171. 178. 179. 180. 181. 182. 183. 184. Arrangement of Blood-vessels in Human Liver (xlvi'ii ) Connection of Lobules of Liver with Hepatic Vein (xlviii Early stage f Development of Liver of Fo W l (lxi.) . Kidney of Fcetal Boa (lxi.) . Section of Kidney of Coluber (lxi ) ' ' ' Pyramidal fasciculus of Tubuli urinif eri of Bird (lxii. ) Section of Human Kidney (on. ) PAGE . 299 . 302 . 303 . 304 . 305 . 306 . 307 . 309 . 313 . 314 . 316 . 316 . 318 • . 325 . 325 . 326 . 327 . 328 . 330 . 340 . 341 . 341 . 347 ) . 361 . 362 . 363 . 364 . 365 . 367 . 397 . 398 . 399 . 400 . 412 . 412 . 413 . 413 . 414 . 419 . 420 . 420 . 421 • 423 . 423 • 424 . 430 . 430 . 430 . 431 XX11 LIST OF ILLUSTRATIONS. FIG. 185. Portion of Tubulus Uriniferus with tesselated epithelium (xcix.) 186. Distribution of Vessels of Kidney (xin.) 187. Corpora Wolffiana, from Chick (lxii.) .... 188. Noctiluca miliaris (lxxii.) ....•• 189. Pelagia noctiluca (xxxiv.) 190. Electric Apparatus of Torpedo (lxxvi.) .... 191. Diagram of Generative Process in Plants . . . 192. Multiplication of Coccochloris by subdivision (xliii.) 193. Conjugation of Euastrum oblongum (lxxiii.) . 194. Conjugation of Eunotia turgida (lxxxvii.) 195. Development of Spores mAulacoseira (lxxxvii.) 196. Conjugation of Zygnema (li.) 197. Generative apparatus of Fucus platycarpus (lxxxvi. ) 198. Tetraspores of Carpocaidon mediterranean (li.) 199. Generative apparatus of Chara foetida (lxxxi.) 200. Generative apparatus of Collema pulposum (xci.) . 201. Generative apparatus of Tremella mesenterica (xcn.) 202. Generative apparatus of Agaricus campestris (lxxvii.) 203. Development of Torula cerevisice 204. Development of Archegonia of Marchantia (lxix.) . 205. Archegonia of Jungermannia (xliv.) .... 206. Sporangia on lobed receptacles of Marchantia (lxix.) 207. Gemmiparous conceptacles of Marchantia (lviii.) . 208. Development of prothallium of Pteris (liy.) . 209. More advanced prothallium of Pteris (liv.) 210. Development of Antheridia and Antherozoids of Pteris (liv.) 211. Development of Archegonium of Pteris (liv.) . 212. Development of Embryo of Poly podium (liv.) . 213. Fructification of Equisetum arvense (lxix.) 214. Generation and Development of Lycopodium (xliv.) 215. Sporocarp of Marsilea quadrifolia (lxix.) . . • . 216. Germination of Marsilea Fabri (lxix.) . . . . • 217. Generative apparatus of Conifers (xliv.) 218. Development of Embryo of (Enotheracece (xlv.) 219. Embryoes of Potamogeton and Amygdalus (xlvii.) . 220. Germination of Zanichellia and Acer (xlvii.) . 221 Constituent parts of Mammalian Ovum (xix.) 222* Segmentation of vitellus of Ascaris acuminata (v.) . 223 First segmentation of vitellus of Mammalian Ovum (xix.) 224. Early stages of development of Coregonus (xcvu.) . . 225. Fissiparous multiplication of Chilodon cncullulus (xxxv.) . 226. Group of Vorticcllw in various states (xxxv.) . 227*. Development and Metamorphosis of Vorticella microstoma (lxxxiii.) . 542 228. Gemmation of Hydra fitsca (xc.) . 229. Generative apparatus of Actinia (lxxxv.) ■230. 231. Development of polype-bud of Campamdaria (xciv.) Generation and Development of Cordylophora lacmtrU (iv.) ■ If i 1 w — ■ 1 I \ j I m_\j \J A. \-/ **-^ «^*r-"— — — — -» 232'. Development of Medusa-buds from Perigminwa (lxxv.) 233. Medusiform gemin» of Cam^autdarla (xciv.) . PAGE 431 432 433 443 445 469 484 485 486 488 489 490 492 495 496 498 499 500 501 502 502 503 504 505 , 506 . 507 . 508 . 509 . 511 . 512 . 513 . 513 . 514 . 518 . 520 . 522 . 534 . 537 . 538 . 539 . 540 . 541 545 546 548 550 551 552 m Crinoid state of Comatula rosacea (xxm.) . \ Development of embryo of Echinaster rubens (lxxv. ) LIST OF ILLUSTRATIONS. FIG. 234. Strobila (or polypoid state of Medusa) propagating by gemmation o ^rroup of Strobila? in process of Medusan gemmation (xxiv. ) JZ' ^ yel °V m ^ °f Medusa-buds from Strobila (xxiv.) . 9 *' ^ evel °Pment ot Medusa from Strobila-gemm* (xxiv.) tZ' ^ emmi P arous multiplication of Cytceis (lxxv.) **». Structure of FeZeMa Zimfant (xlvi ) 240. «■■•■•-■ 7 ■ 242. Mpinnana asterigera, or larva of Starih ( L xi7) -lorn, in.) VT~ " r ! 0r g a ^isation des Vers (K^ a a • Zool., Tom. vn.-xn.) *"*' (Ann ' des Sea- Nat., 3« Ser., xii. ) (See also xxiii. ) ™: Sr: A rr *»«* *~» s: *s £*»-* *•' rf *• *<*- **. * a . xiv » .' (PMl Trans -' 18 *2.) xvn i!f™f' Brid <*™« Treatise „„ 8e.lo OT . (Ann. des Sci. Nat., xxn. xxiii. C«te, Histoire Oenerale et Particuliere du n* , Organises. lere du D ^eloppement des Corps Couch (E.), On the Metamorphosis «f «. t. Memoires ur les Mnii, pa a . lvl °ilusques. fi egne Animal n^u; ■^^^^i m^m ■r w^^^m XXVI TREATISES AND MEMOIRS REFERRED-TO IN ILLUSTRATIONS. xxv. Darwin, Monograph of the Cirripedia. xxvi. UOrbigny, Cours Elementaire de Paleontologie. xxvii. Dumeril et Bibron, Histoire Naturelle des Reptiles. xxviii. Edwards (Milne), Histoire Naturelle des Crustaces. XXIX. XXX. Sur les Ascidies Composees. Snr la Circulation chez les Annelides. (Ann. des Sci. Nat., XXXI. XXXII. 2 e Ser., Zool., Tom. x.) Sur le Developpement des Annelides. (Ann. des Sei. Nat., 3 e Ser., Zool., Tom. in.) Sur la Circulation chez les Mollusques. (Ann. des Sci. Nat., XXXIII. XXXI v. 3 e Ser., Zool., Tom. vni.) Recherches sur les Polypes. Cours Elementaire de Zoologie. (See also xxin.) xxxv. Ehrenberg, Die Infusion sthierchen. XXX VI. - Des Leucthen des Meeres. (Abhaldlungen der Kon. Akad. der Wissenschaften zu Berlin, 1835.) xxxvn. Flourens, Memoires d'Anatomie et de Physiologie Comparees. xxxvin. Forbes, Monograph of the British Naked-eyed Medusae. History of British Starfishes, &c. On Beroe pileus. (Ann. of Nat. Hist., Vol. n.) XXXIX. XL. xli. Goodsir, Anatomical and Pathological Observations. xlii. Harting, in Mulder's Chemistry of Animal and Vegetable Physiology. xliii. Hawaii, History of British Freshwater Algae. xliv. Hoffmeister, Vergleichende Untersuchungen der Keimung, Entfaltung und Fruchtbildung Hoherer Kryptogamen. Sur la Fecondation chez les (Enotherees. (Ann. des Sci. XLV Nat., 3 e Ser., Botan., Tom. ix.) xlvi. Hollard, Sur 1' Organisation des Velelles. (Ann. des Sci. Nat., 3 e Ser., Zool., Tom. in.) xlvii. Jussieu, Cours Elementaire de Botanique. xlviil Kiernan, On the Structure of the Liver. (Phil. Trans., 1835.) xlix. Kolliker, Entwickelungsgeschichte der Cephalopoden. Surle Developpement des Tissus chez les Batraciens. (Ann. L. des Sci. Nat., 3 e Ser., Zool., Tom. vi.) LI. Kutzing, Phycologia generalis. lii. Leidy, On the Comparative Structure of the Liver. (Amer. Journ. of Med. Sci., Jan., 1848.) . On Articular Cartilages. (Op. cit., April, 1849.) Leszczyc-SuminsH, Entwickelungs-geschichte der Farrnkrauter. LIU. LIV. lv. Leuret, Anatomie Comparee du Systeme Nerveux. lvi. Lister, On Tubular and Cellular Polypi, and on Ascidise. (Phil. Trans., 1834.) lvii. Mantell, On Iguanodon. (Phil. Trans., 1848.) lviii. Mirbel, Sur la Structure et la Developpement de la Marchantia poly- morphs. (Nouv. Ann. du Musee, Tom. in.) lix. Mohl, Vermischte Schriften botanischen Inhalts. lx. Muller, (Heinrich), Zur Hectocotylus Argonautae. (Siebold and Kbl- liker's Zeitschrift, June, 1852.) TREATISES AND MEMOIRS REFERRED-TO IN ILLUSTRATIONS. XXVI] LXI. LXII Mill LXIV. hXY. LXVI. IX VII. KVIII. LXIX. LXX. LXXI. LXXII. IiXXIII. LXXIV. LXXV. LXXVI. LXXVII. LXXVIII. LXXIX. LXXX. MXXI. LXXXII. I'XXXIII. I'XXXIV. i and - Vegetable Circula- tion. (Trans, of Soc. of Arts, Vol. xxxix ) 2 on e M u r n B r in ' its , structure ' physioi °^ aDd D « I £ f ^f^® of Vorti ^a. (Siebold and Kolliker's Zeit- scnntt, Band in.) Strauss-Burckkeiru, Considerations Generales sur V Anatomie comparee des Animaux Articules. ^Anatomy of Actinia Coriacea. (Trans, of Phil, and Lit. Soc. of Leeds, Vol. i.) TW Sur les Antheridies des Cryptogame, (Ann. des Sci. Nat., o ber., ±5otan., Tom. xvi.) Thwaites, On the Conjugation of the Diatomacea, (Ann. of Nat. Hist., 1st Ser., Vol. xx., and 2nd Ser., Vol. i.) Tiedemann, Anatomie der Rohrenholothurie, &c. Sur le Developpement du Cerveau. Trembley, Memoires pour servir k l'Histoire d'un genre de Polype d'eau Tulasne, Sur les Lichens. (Ann. des Sci. Nat., 3 e Ser Botan T™ „ ^ Sur les Tremellinees. (Op. cit., Tom. X "x.) ' " ^ (Ann. des Sci. Nat., xciv. xcv. Unger , Recherches sur TAchlya prolifera. & feer., Botan., Tom. n ) V Acad - R °y- de Bruxelles, Tom. xvn.) (M6m^ c t C t t" T **„**»>*" de la ™° d'Ostende. v « .acaa. Koy. de Bruxelles, Tom. xviii.) , XXvili TREATISES AND MEMOIRS REFERRED-TO IN ILLUSTRATIONS xcvi. Van Beneden, Sur le Developpement et 1' Organisation des Nicothoes. (Ann. des Sci. Nat., 3 e Ser., Zool., Tom. xm.) xovn. Vogt, Embryologie des Salmones. xcvin. Recherches sur l'Embryogenie des Grasteropodes. (Ann. des. Sci. Nat., 3 e Ser., Zool., Tom. vi.) xcix. Wagner, Icones Physiologies. c. Wallick, Plants Asiatics Rariores. ci. Willis, On the Organs of Voice. (Trans, of Cambridge Phil. Soc, Vol. iv.) en. Wilson, Anatomist's Vade-Mecum. •I ] * ** . .. , . ^ ■ CHAPTER I. ON THE GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 1 . There are few tilings more interesting to those who feel oleasu r* 111 ^ordinary advancement of knowledge at the present of Bint" 1 Q rapid P T reSS ° f P hil0S °P^al views in every department of Biologica Science ; the pursuit of which has until recently been made Xrs t? s v xclusi ? ly ' in the mere coiiection and -cLXC f ttyZeXntTl "* ^^ a * the discove ^ of the ideas of which beyonl the reach 'ST**™*'' r ^ ° f ^ ^ lon « COnsidered atteCw n f 1 T an m 7 eStlga ; t1011 ' and the mind shrank fr om consWW ^ F* lts . com P lex a » d var ied phenomena, which, though CZZ^tor* e ™ tl0n ' mU t b , e reduCed to ^eir'simple'st form, WeveTVf S r T + mn | C f n be f ° Ullded U P° n them - Xt ^ rec ^ded many of the IT ' a ' T hll f. c r tem P latin S the ^plicity and har- med m the ^ ^J^ the TJniverae is S 0vemed > as ^i- distant and 1 ^ ^ gl ? ntic mind discovered between the Noughts 5leT + rr nneCted J"*** ° f the solar ^tem, his ^ondtfuf Zl a ^ ° rgamSed Creati ° n > and refleeti 4 t^t the less a deoree Z Z* i ^ arra ?g emen t ^ich it exhibits, present in no from OmSoS n i r T ^ ? r t r ^ perfeCti0n Which caii result t^re rf2S? ' ?l remarked, « I cannot doubt that the struc- que dlci ™ + ? gOVe / ned ^ P^ciples of similar uniformity." (" Idem- «WW» in . umformit ate ilia, quse est in corporibus animalium.") fflo-h* « ™^ uvie r m his eloquent discourse on the revolutions of the 9 au? d not Natu ral History some day have its Newton?" Wl A1 ;hough the labours of the Naturalist and Comparative Anatomist have not yet unveiled more than a small part of that -eneral r^T 2£ f4or s r r y of wM r h r y p t rhaps be reserved tiSX^f = y ssj^^^^aX^ °s a ! solid fo - da ^rrd firmation. Several of these laws , r^l 1 ' ■ ^ 5 eCeiVmg fresh Con " range, and interesting from hi HZ ^ 1^^/™™ their extensive they frequently h^d ln^ T^ V ^^^ ° f the reSults to ^ich forced, and mlj^t^ffi ! ^f ^ may somet ™es appear vestigkon willXerJirl ^,7 f sim P licit y of Nature, f^fm- ^^Ij^^Z^ *» difficult 7 J «» apparent than ^ proportion J^^Z l T ° Ur ° Wn P re J udlces > a » d diminishing 1 won as we fax our attention upon that combination of unity of B ... 2 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. united with harmony of forms, so remarkable in the animated world. ^ 3. In comparing phenomena of any kind, for the purpose of arriving at a principle common to them all, it is necessary to feel certain that they are of a similar character. Indeed the sagacity of the philosopher is often more displayed in his discovery of that relation amongst his facts, which allows of their being compared together, than in the inferences to which such comparison leads him. The brilliancy of Newton's genius was shown in the perception, that the fall of a stone to the earth, and the motion of the moon around it, were comprehensible under the same law ; not in the mere deduction of the numerical law from the ratios supplied by those facts.— In the sciences which have Life for their subject, the apparent dissimilarity of the facts which are made the objects of com- parison, often prevents the true relation between them from being readily detected. Here it is that the mental training which the previous culti- vation of Physical science affords, becomes peculiarly valuable to the Physiologist. The most important part of the process of Induction," W fv_/ M . X_A .11. ^**r ^Mk -*■ r**^ ^^ » — ■** says Professor Powell, 4 ' " consists in seizing upon the probable connecting relation, by which we can extend what we observe in a few cases to all. In proportion to the justness of this assumption, and the correctness of our judgment in tracing and adopting it, will the induction be successful. The analogies to be pursued must be those suggested from already-ascer- tained laws and relations. Thus, in proportion to the extent of the inquirer's previous knowledge of such relations subsisting in other parts inference that before him. limited observe the connection between one class of physical truths and another, will almost unconsciously acquire a tendency to perceive such relations among the facts continually presented to him. And the more extensive his acquaintance with Nature, the more firmly is he impressed with the belief that some such relation must subsist in all cases, however limited a portion of it he may be able actually to trace. It is by the exercise of unusual skill in this way, that the greatest philosophers have been able to achieve their triumphs in the reduction of facts under the dominion ol general laws." 4. The first group of phenomena encountered by the Biological student, is that presented in the many hundred-thousand diverse forms of organic structure of which the Animal and Vegetable kingdoms are made up \ and it is necessary, at the very outset of the inquiry, to settle the prin- ciples upon which these are to be compared. In many instances, it is true, there can be no room for hesitation ; the general type of conforma- tion' of two or more organisms being obviously the same, and the differ- ences in detail never obscuring the resemblances between their component parts. But this holds good to only a very limited extent ; and we are soon forced to recognise such essential differences, alike in the general tvpes of conformation, and in the form and structure of the component parts that we feel the need of some guiding principle according to which we mav arrange these phenomena for comparison. — Now from the time of Aristotle downwards to the commencement of the present century? -k 1 1 Connection of Natural and Divine Truth," p. 33. a o s: Hi P o o a til t t: t: t e t: wj ml P S" it o b p 1X1; if o tl b hi Vi se: thl su: w \. BASIS OF COMPAKISON OF PHENOMENA. 3 ty rig at is |ts ? to us Ihe ed he |iiy ti- lie 11 ng 11. of er- ,he ,rts k in to Ions ive Ithe da of He ot nt, nic in- is ma- or- ent are leral ent hicb ime ^tlSto 8 ^ ^ haMt ,° f regardin « sMarit ^ * Vernal form Parts. But akhoS?V " f^f^ the analogies between different perfectly cLre^^^ ^ ° f e8t ™^ the character «f organs is that is, when we 1^ ^ ^ COnsidered as ^~m to J rtnl^- formed oy them TtotXT^ ^ *" C ° nditi ° nS of the f ™ cti on P^" ^m,a£ aTco7d1nl^ ^^ in Search of the ^ «/ it « frequeX fonnd g t W I ***** ^^ haS taken P lace ^ ^ form, a^Ki 11 !** organs which are not nnlike in external from e Wit entirllv Tf^T^ Wt r S in the S ^ stem ' originate ^ilar : wh£s t on it ^ I' ?* "* theref ° re f^mentally clis- Httle or' nTr semo Jance to Z W.' ^T "^ at first ^ P^ent Ptoses in tCc ^my mT^^^^^^ Afferent mental components. 7 modifications of the same funda- or LL f TnTel r \Tt e h thin "ST 7 fT* f ** «*»■ ° f SU PP ort observe a coinmST ot wl T* Am ° al f ™ e fomi ^d, we shall form, concealTngT total dto££ SitJTf t^ ° f eriamal character. Amongst all t WW v ? st ™eture and of essential respiration, w en^oull ^T?a ^ ^ ada P ted for atmospheric resistance of this eCent beZ.f + f ^ V* 8 6Xtent > in which the and even *moT*^J™°™t ^P* meanS ° f Progression ; tion of locomotio^n^rT ' ? 6re are mst ances in which the func- tme ^CSSit fet! Up ° n the Same a W Wherever member "serv f T&Z^tTL* ™ mod * Cation of *» anterior mode in which th P »™™+ • ' but there is considerable variety in the ^e required tea $^ ? instructed. Thus, in the Bat (Fig. 2 e) the akL^i 8 ^^ 8 ^ 6106 of ,t^™g iB formed by an extension of ^e largest part ^T+v W \ of whlch those of the hand form by far ^tremi t ; Lf is alched tT^T ," , ertendfid als ° from the poster or the tail 4ere oL ISst Vt ^^th °f * he Wk > as wel1 a * to ^g is formed by the skin Id Mt ^ <*> *' B) ' ° n the «**"** the member alone ; SdwSe^? T^ 68 , attaclied to **» anterior paratively slightXr ee those ofT ^ ^ are de ^loped in a com- ^pport of the expaC'n ^om X + m ^TV*^ the P rinci Pal l * «eems that the winTof tlST^ ? preServed of **» Pterodactyls, ^er the whole memW as iftL e ^aordmary animal was extended, not tmt over one of +ZTT ? e -? ird ' nor over ^e hand as in the Bat Flying-fish. may'be reffardp? t a i F - g * ^' XXi ™ e . ^* W^, again, the pectoral fins that the aWl ^ \S ^ ^ lts wm S s (^ough it does not appear on ^-IrXi^,!^! 1 ^ moment o?^ ° f ^ S these fins evidentir renrewft \l„ I ■ moment of quitting the water\ • bnt the bones of The a^lnd tei™ l memWl ? ° f . «* - V-tataS i hand very •expanded, and joined immeZll SCa . y develo P ed ? ^hih Cerent structuri prevll fl t J ' + f rt W6re ' to the ^nk. _ serve rather to «^ th P e ^1^ , n \ tllose ™perfect wings, which ments through the air than T T^ P ° SSess them ' in their move- 4 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. safety from considerable heights : in the Draco volans, on the other hand, the wings are affixed to the sides of the back, being supported by pro- Ficx. 1. Pterodactylns eraviirosiri* . longations of the ribs, and are quite independent of the extremities. Here we have still the same function and general form ; but it would evidently be absurd to say that the organs are of the same structural character.— A still greater departure from the type with which we are familiar among the higher animals, is presented by the wmgs of Insects ; for these are formed by an extension of the superficial tegumentary mem- brane over a framework that is not derived from an internal osseous skeleton, but is an extension of the denser subjacent layer of the external integument ; and this framework is penetrated, throughout its ramifica- tions, by ' tracheas' or air-tubes, communicating with those of the interior of the body, and also (at least in the early state of these organs) by vessels or passages for the circulation of blood. As regards their essential struc- ture, in fact, these wings correspond closely with the external respiratory organs with which certain aquatic Larvse (as that of the Ephemera) are provided ; and hence they have been not inappropriately designated by Oken as < aerial gills.' * They may, in fact, be regarded as an excessively developed form of those external appendages of the lower Articulata, which are subservient to locomotion and to respiration j ointly ; and it is a very interesting example of the similarity of modification which very different plans of structure may undergo, when a common purpose is to * The attempts of a generation of Entomologists now passing away, under the influence of the erroneous idea already referred-to (§ 4), at bringing into comparison, as similar organs, the wings of Insects, and the anterior members of the flying Vertebrata, can now only excite a smile on the part of the Philosophical Anatomist. Such attempts, however, have exercised a most injurious influence on the progress of science, by drawing oil the attention of Naturalists from the true method of philosophical research . t k d t: o: s A tl stl is PI id wl P fol o o: t adl ha ( BASIS OF COMPARISON OF PHENOMENA. h .es. aid al re s; m- us ca- ior els luc- ■ry lare by ely ta, is ery to nice ilar InoW :ver, the be therp rfmJilk • , 8 Ui tne - Dira ' as wel1 as m tnat ot ™e Insect, fraxnewk which l^^t^™ * **" "^^ ^^ ^ ^ Wd^V^^Tif might readi1 ^ be adduced from ^ Animal SSe To^V Ve S etable r rld aff0rds them in eve * neater abun- throush wW^ a "P? ry + T plG c f ej ~ the ex P a * d <*i foliaceous surface of S St f la ?V btamS / r0m the a ^phere the principal part which are »Z 7 * °! ^ ^^ th0Ugh USUa11 ^ afforded V ^e leaves, SlSf *? ^ ^ 1S deVel ° ped f0r tbis express purpose, i i^oo&l ™ T ams It and succulent ; wMst in ma »y of *** thiHLTrf T- ^ M ^ m 1 th6 sub " a( l uatic leave * of the tfoyfifa™ of s a k S n 7 1 Xt - 1S g T n V h 7 e lami » al compression of the petiole or leaf- Pose of !?' agam ' t 6 te f n \ WhiGh is an ° r ^ n developed^ for the pur- lin f h rP ° rtm f ^ Pknt ^ twinin g rou » d some neighbouring £L is m the Vvne a transformation of the peduncle or flower-stalk in t£ £> a prolongation of the petiole or leal-stalk, in the 6W & t 'a W location of the stipule, and in tffonow* the point of the leaf iS • ^ V tbe K Smgular S enus Skophvrim, it is actually the poinl of the petal which becomes a tendril and twines round other parts. P Wtion Zd^Tt 7 ? l6Ct ^ 6X r ple ° f *?*»** of Vernal con- Fig. 2. B 1) E F , — 7 /S3> "/' \% tt\ ^i Different forms of Anterior Member • ^ Fish; B , Bird; c, Dolphin; n,Deer; e, Bat; F , Man. SS£f sHghStst^ "T e :? f lif \ N ^ Comparati - **»* ("Fi- 2 a WW? • atl °* m admitting that the pectoral fin of a Fish ^ W 2, A), the wmg of a Bird ( B ), the paddle of a Dolphin («,), the fort 6 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. leg of a Deer (d), the wing of a Bat (e), and the arm of a man (f) are the same organs, notwithstanding that their forms are so varied, and the uses to which they are applied so unlike each other. For all these organs not only occupy the same position in the fabric, but are developed after the same manner j and when their osseous frame-work is examined, it is found to be composed of parts which are strictly comparable one with another, although varying in number and in relative proportion. Thus, ywhere without little developed as not to intervene between the scapula and the bones of the fore-arm; next we have the radius and ulna, whose presence is always distinguishable, although one of them may be in only a rudi- mentary condition; then, beneath the wrist-joint (through which a dotted line is drawn in the figure), we find the bones of the carpus, which are normally ten in number, forming two rows, but which may be reduced by non-development to any smaller number— three, two, or even one ; next, we find the metacarpal bones, which are normally five, but are sometimes reduced among the higher vertebrata to four, three, two, or one, whilst in Fishes they may be multiplied to the number of twenty or more ; and lastly we have the digital bones, of which there are normally five sets, each consisting of three or more phalanges, but which are subj ect to the same reduction or multiplication as the metacarpal. — It is entirely from the differences of conformation which these osseous elements gradu- ally come to present in the course of their development, that those special adaptations arise, which fit their combination in each case for the wants of the particular species that possesses it ; enabling it to be used as an in- strument for terrestrial, aquatic, or aerial progression, for swimming and diving, for walking and running, for climbing and flying, for burrowing and tearing, or for that combination of refined and varied manipulations Man wherewith 8. We : Respiratory functional An analogy, and organic correspondence, or Iwmology* into clear view- uninstructed observer would scarcely perceive any resemblance between the gills of a Fish (Fig. 145), and the lungs of a Quadruped (Fig. 154), or tufts tubes ramifying through the body of an Insect (Fig. 149); and those who formin be led to deny that any real analogy could exist. When * In earlier editions of this work, the terms functional and structural analogy were used to express the mutual relation of parts, on the one hand as instrumental structures, on the other as fundamentally or organically correspondent. It will be found con- venient to limit (as Prof. Owen has done) the use of the term Analogy to functional resemblance, and to employ Homology as indicative of structural correspondence. Thus by Analogue we now understand " a part or organ in one animal, which has the same function as another part or organ in a different animal :" whilst by Homologue is implied "the same oro-an in different animals under every variety of form and function." (Prof. Owen's "Hunterian Lectures, "Vol. I. Glossary.) Thus, for example, the wing of an Insect is the analogue of that of a Bat or Bird, but not the homologue ; whilst the latter is homologous with^the arm of Man, the fore-leg of a Quadruped, and the pectoral fin of a Fish. HOMOLOGY AND ANALOGY. 7 t ,1 ■s d g s t Y n P- o Jt of ire •ri- al us Ime led of. an iter fin the function is investigated, however, with the structure which it requires tor its performance, it becomes evident that, in order to hring the circu- lating fluid into the due relation with the atmosphere, all that is needed is a permeable membrane, which shall be in contact with the air on one side, and with the fluid on the other. And this key, applied to the examination of the several forms of respiratory apparatus which exist m the Animal kingdom, shows that they all possess the same essential nature as instrumental structures, and that their modifications in par- ticular instances (which will hereafter be specially described) are only to adapt them to the plan and conditions of the organism at large. There is therefore, functionally considered, a relationship of analogy amongst all these organs; although they are not really the homologues of one another. Thus, the gills of the Fish, and the branchial tufts of the feabella, are external prolongations of the tegumentary surface, whilst the tracheae of the Insect, and the lungs of air-breathing Vertebrata, are internal reflexions of that surface; and farther, the two former sets of organs, as the two latter, differ from each other in regard to the part of the surface from which the prolongation or inversion takes place. In the Perennibranchiate Batrachia, moreover, both lungs and gills are present; and their essential difference of character is most apparent whilst their correspondence as instruments of the same functional opera- tions is equally evident. Further, in the air-bladder of the Fish, we Have an apparently anomalous organ, the only known use of which is to assist in locomotion; yet when a comparison is made between its most developed forms and the simplest pulmonary sacs of Amphibia, no doubt can remain that it is to be regarded as a rudimentary lung ; and the sraay of its development leads to the same conclusion. (See chap. vi. ) y. It would be easy to adduce numerous parallel examples from the egetable kingdom; wherein organs which correspond in structure, con- ations, and development, and which are therefore homologous, are oserved to assume the most varied forms, and to perform the most umerent functions. It will be sufficient, however, to advert to the well- ' Known tact, that the underground < creeping roots' (as they are com- monly accounted) of the Couch-grass, the subterranean < tuber' of the potato, the < rhizoma' or < root-stalk' of the Iris, the solid < cormus' of the * J-olchicum, the < bulb' of the Hyacinth, and the < runner' of the Straw- berry, are not less truly sterns, than are the lofty trunks of the Palm or Ulm, notwithstanding the variety in their form, texture, and mode of growth; for they all constitute the ascending axes of the Plants of which they respectively form part, and have the power of developing the loliaceous appendages which become leaves or flowers, whilst the radical fibres, which constitute the essential part of the roots, grow downwards from their base. the functional with perfect propriety found naiWl nr, A -n+L • ., # - jyvw*, kjl uiie organs respectively com- pri! P ^ ^f lty aS mst ™ments adapted for a particular purpose For example, we might estimate the respective force with which Birds" Et ; fi 1 Wt V° + ? ld *> e Propelled through the air, by ascertaining the superficial area of their wings, and the energy and rapidity with which animal oi Plant, by the extent of surface through which the nutritive 8 DEVELOPMENT fluid comes into relation with, the atmosphere, by whatever portion of the fabric that surface might be afforded. But the Philosophical Anatomist, who seeks to determine the organic relation of these parts, must first consider their internal conformation, and examine into the structural elements of which they are composed. In the cases just alluded to, he would find not merely the osseous elements, but the muscles, nerves, blood-vessels, &c, presenting essentially the same disposition in the arm of Man, the fore-leg of the Quadruped, or the wing of the Bat or Bird. But on passing to the Insect, he would encounter, as we have seen, an entire change in the plan of structure; the same purpose being fulfilled by instrumental means of a very different order, corresponding, in fact, to those which in the Articulata generally are made subservient to the respiratory process- — The next step in the determination of homologies, is to trace the connections of the organs compared, which frequently enables the real nature of parts to be recognized, which would be otherwise obscure. For it is a principle of very extensive application, that similar parts are connected with similar parts, in different animals of the same type. Thus, we never find a hand or foot springing directly from the spinal column of a Vertebrate animal; the connection being always established by other bones, which, whatever may be the variety in their size and shape, are never wholly wanting. Hence, where we find, as in the Fish, the hand excessively developed, and no external trace of an arm or fore-arm, we expect to find it supported internally by a radius and ulna, and these again to be connected with the scapular arch by the intermediation of a humerus. Now the bones of the fore-arm are generally distinctly deve- loped, whilst the humerus very commonly coalesces with the coracoid, so that its presence might be easily overlooked; yet even this is found in certain species to be present as a separate bone.* — Great assistance, again, in the determination of the homologies of organs, is afforded by the examination of transitional or intermediate forms. Thus, it has been by the regular progression exhibited in the structure of the pulmonic apparatus, from the simple, closed, undivided air-sac of most Fishes, through the higher forms which this organ presents even in that class, and through the various phases which it exhibits in the Perennibran- chiate Batrachia, that the homology of the swimming-bladder of the Fish with the lung of the air-breathing Yertebrata has been established. In like manner, it has been by tracing-out the intermediate forms of the bones of the extremities (Fig. 3, b, d), that Prof. Owen has succeeded in proving the complex limbs of the higher Yertebrata, to be homologous with the simple rod-like members (a, c) of the Lepidosiren (Fig. 150); whilst these last serve as the connecting-link, whereby the homology of the sca- pular and pelvic arches with the haemal or visceral arches of other verte- bral segments is indicated; the bones of the limbs being at the same time shown to be homologous with their ' diverging appendages' (of which we have examples in the backward projections that spring from the ribs of Birds), and the scapular arch with its anterior members being thus found * See Prof. Owen's " Lectures on Comparative Anatomy," Vol. II. p. 120. The two bones supporting the Fin in Fig. 2, a, are considered by Prof. Owen to be elongated carpals, not (as usually supposed) radius and ulna. If this be the case, the member should have been so placed in the figure, that the dotted line which marks the place of the wrist-joint should have passed above instead oihelow them. v MEANS OF DETERMINING HOMOLOGIES. 9 tlot? t} T COm P letion of the occipital segment, whose centrum and neural drcn enter into the composition of the cranium. So, again, the identity of Fig. 3. 13 o,> J^^t»^*^^^H5& f T^ **,<»*«>* vertebra of -a, posterior view of the peMe wrtebra of V.nf^ f P verteb ™ ?f Amphmma didactylum: tebra of Proteus anguiZ P £ the ^SSl dSS^Tfh^f V ^*™* ™ of the pelvfe ver- sponding parts ; c, centrum ■ n neSollf T^ i following references indicate eorre- occipital Vertebra or scapula? K? fiZ^s of tK/V,' P 1 ^ ^ 8 ofthe bone ; a, 53-57, diverging anpendaeetoffhT^S+fi <■ £ ^P 1 ^ vertebra, or coracoid rapophysis of the pelvic v ertebra o fu?ac bo n? «l \ er tebra, or anterior limbs ; pi, 62, pleu- rae bone , „, 6 i 69 , di^SS^ ^T^^ofpSS^^ » by the transitional true +i discover JUTrf?C£i ottn^ ° f f 7^/ <*? iS 'Tike sent? +„ +T. llu ";V 10 to°us organs, under whatever forms they mav WP hlSwV* P er . ce P^ of that great general truth, which is, perhaTs^ highest yet attained m the science of Organisation, and which^s ten ^ •1 K * 10 DEVELOPMENT far from being fully developed; that in the several tribes of organised formed an independent model, and presenting a type of structure peculiar to itself ; but that we may trace throughout each assemblage a conformity to a general plan, which may be expressed in an ' archetype ' or ideal model,* and of which every modification has reference either to the peculiar conditions under which the race is destined to exist, or to its relations to other beings. modifications themselves present a conformity to a plan of less generality ; those next in order to a plan of still more limited extent : and so on, until we reach those which are peculiar to the individual itself. This, in fact, is the philosophic expression of the whole science of Classification. For, to take the Vertebrate series as our illustration, we find that Fishes, Reptiles, Birds, and Mammals agree in certain leading features of their structure, which constitute them vertebrated animals \ but this structure is displayed under diversified aspects in these classes respectively, which constitute their distinctive attributes. Thus, of the general vertebrated type, the Fish presents one set of special modifications, adapting it to its peculiar mode of life ; the Reptile, another ; the Bird, a third ; the Mammal, a fourth. So again, in each of these classes, we find its general type pre- in the respective orders; thus, for example, the Reptilian type exhibits itself under the diverse aspects of senting subordinate modifications Man v ^ l ^ wv ..„ „_, , Mammalian f the "Whale and the Bat, the Sloth and the Deer, the EJ . & v~, ««v, «^« QWW ~~ Monkey, the Ornithorhync families, in accordam the subordinate or more special modifications which the type of the order presents ; every one of these families displaying the type of the class and own Each family consists of genera, in every one of which the family type is presented under a some- what diversified aspect. Each genus is made up of an aggregation of species, which exhibit the generic character under a variety of modifica- tions ; these, however slight, being uniformly repeated through successive Lastly, each species is composed of an assemblage of indi- viduals, every one of which repeats the type of its kingdom, sub- kingdom, class, order, family, genus, and species, through its whole line of descent. 12. Thus, in assigning to any particular being its place in the Organised generations. unknown bodv to be brought for our We business is to ascertain whether it be an Organised fabric, or a mass of This is soon discriminated, in the majority of cases, Inorganic matter. by an appeal to those most general characters which distinguish all organ- ised structures from inorganic masses ; and the next question is to deter- mine its Animal or Vegetable nature, by the aid of those characters which are not common to both, but are distinctive of each respectively. We will suppose the Animal nature of our unknown body to have been ascertained; the question next arises, to which of the four sub-kingdoms * For an admirable exposition of this doctrine, as it Vertebrated Animals, see Prof. Owen's treatise on " mi - the Vertebrate Skeleton." The respects the osseous system of Archetype and Homologies of i^^Ml WX t l DIFFICULTIES IN DETERMINATION OF ARCHETYPES. 11 Jo ta^^rl^L -S^L 5" to -*, — aed * » appeal being which are less general than the preceding/ not structures, nor «* f„ oil ~™~^ but being Then having animal, ^thel* I^tf M5?5E* A ,^*d, or Radiated Naturalist by characters of ^liw ' v! XNa ™ ls t would determine its order family b! fU^s 1 IT ^ WlUCh T e peCuliar to that order 5 *» mediation *5&Jl^5,« ^ Tf Hmited; its S enus > fc y those genera ftyK lastW ^ T P f Sented b ^ the several most specialoT tlltZu IV® TT ', hj <**™**»* which are the ^ oiai °* an, that is, which are limited to that race alone 4V^^^ it -old Comparatively -e y should have all the LmsSorT us^bv thf ° f ^ ^ W ^ Philosophical Zoologist seefe to eduo P wl, n f • comparison of which the in practice it has often ^f^S^^^Tf ?? whole - B ^ shall be considered as whilst the W bale progression. But the SlS'^ fo rm habitation, and mode of Bat amongst the M^ZStS^J^ h T*f™ * ^^ the distinoru namely its viviparous aid placenta! gTeXf & *7 *** 5 BirdS ' of its young by lactation, its coverm/of W S, * ^equent nurture mouth, its diaphragmatic resmrltSw! V ^'i / d ? tal arma ^e of its many other peculiarities o ^Zw;i ^ ^eloped cerebrum, and o the class Sf Birds mt^W^^^^^ -semblance type to an aerial life. So M^^^ its *?^ habits, Whal •T ""^giuaaes it irom that of Fish e* • ™™.i -I T asH > ana wnich Jts complete double oiroal5^43^^?? h «rio respiration, and subsequent lactation its well I rW?^ j ' , lts vm P a rous generation ^any othe'r peculiarity ^-^^W^^^*?*^ * 1 " 1 a » d who possess but a smattering of VnnW i determmatlon u eas F to those another case. in wS !f^il Zoolo S lcal ^owledge. But we will tat. great Mast value of characters. fundamental vaiue ot characters. Following too closely the 3^ e * e ^ion of the the teeth (which are valuable in so far onlv u ^hev ^ aff ° rded % general i an of conformation), Cuvie^nW ^iT™ aS a ^ *> the m the first instance as a subdivisknof hi „ . *T"^ ^«iMno«« when he subsequently raised th^+-f + ? hlS 1 0rder Carnwia; and even them a position mte^meS ^ <* a ^tinct order, he ££ -Likewise, on account of th* *i, ^ e Carn ana and the Ro<% and # Mussel tt th r ni 7 r P e ^"^^o^SSw S?S5S^?V&S2^ study ofTe ^ana W o^ ^t^ t^ ^ he had been led ^ the theiJstron SSf 2 the T? J 1 ?*** 1 !* of ? e ^Us, to recognize their bodii^^^J^? 1 * 64 , Smes - For he Perceived that into a lo^^J?^^^"^ and P resent ^cations of division with a pX of Til 1 TT 11 ° f S6 f mentS ' each of them foniiahfid (Fig. 5 ) tosher with kS^T^' ^ r^ he °" to he region: wit, oW J ?S. h * f °™ d their ^ to lie in the dorsal region; whilst along their ventral region he detected the double series are ^I^Sft,^ ^ relati ° nS t0 the ^lluscous ferred the CiSipeds t^ tll^ ^ 1°? P 8 ?* 10 " 1 " *°de of life,-trans- transference^rlVe mantfeStv ?f T > "* ^ ^^ of this Mr. J- V. Thomns™ in SSfA ? G S scovei 7 (first announced by are free-movirSak S f } ' ti ^ ^^P^ m their early state Crustacean W 3 + t !°» formab e m a11 essential particulars to the racter a ?W ^ ' d that they ^ attain their adult form and cha racter after a series of metamorphoses, which progressive^ ^It? teZTe^ an l^ a t- distance" from it, and ^C^ltl^Zlt^Z tend to evolve the peculiar conformation that distinguishes t n I OW v 7 , group, adapt the animal, in each of its stages, to SZ^^i^* 1 ? 6 * Mr. Thompson, with have subsequently »WfaX ^2^ whi <* they difference between the early fasX 7 ^f 6 1S n ° ess ential Cirrhipeds; but that both Ire ^ active 1 ttl w- *" P edu ^^ed three pairs of legs and a pair of oTl T™** (Flg ' 5 ' A )' Pressing covered with an expanded shield TT, d ^ *** ^ the bo ^ Crustaceans, so as in no eTentkfLS 1 + l-^J Ento ^tracous Cyclops (Fig. 60). After go W £0 S ^7 *T the larva of / gomg through a series of metamorphoses, one Zoological Researches," No. III. 14 ORGANIC STRUCTURE AND stage of which is represented in Fig. 6, B, these larvae come to present a form D, which reminds us of that of Daphnia, another Entomostracous Fig. 6. ^ Development of Balanus balanoides; — a, earliest form ; — b, larva after second moult ; — c, side view of the same; — d, stage immediately preceding the loss of activity; a, stomach (?) ; b, nucleus of future attachment ( ? ) . Crustacean ; the body being enclosed in a shell, composed of two valves, which are united along the back, whilst they are free along their lower margin, where they separate for the protrusion of a large and strong anterior pair of prehensile limbs provided with an adhesive sucker and hooks, and of six pairs of posterior legs adapted for swimming. This bivalve shell, with natatory thrown off; the animal then attaches itself to its head, a portion of which Fig. 7. Comparison of Leucifer, a Stomapod Crustacean, with Lepas ;— in the former, a, the abdo- men, which becomes rudimentary in Cirrhipeds, is represented in outline ;— in the latter, b, the antennae and eyes, which really exist in the larva, are represented as if they had been retained and had continued to grow ; m marks the position of the mouth in both. - T H DIVERSITY OP PRIMARY TYPES OF ORGANISATION. 4 15 wffilL TT? 7 el0B S ated ato the peduncle of the Barnacle (Fig. 7), the W t w - nUS Xt ex P ands int0 a *>road base or disk of adhesion ; over the rZ?Z S^T? Sends backwards a prolongation which arches occurrence atnl+lf n y /° *? com P letel y to enclose it (no uncommon HdaZ rntotJ ? H- US i ^ W the exterior la ? er °f this » conso- segments are evol IT!— 6 ^ j , wHlst from the ot her thoracic W ani^ri^- ^ ®u P + T ° f T ? irrlli wHch are characteristic of DeeiTWf- VI 6ir adult st ate— Hence, whether we consider the stituting ^rt atSti ?of + r Ct ^ OT Whether We re S ard * as c on- longer LyTuelt^ihl^oV^f ? rustacean class, there can be no the latteTaTd tat w! °/f bl P eds b ear ™Y extremely close affinity to or beyond Twf » 7 I be pl T d near its borders (whether within adapteTto ^ilZ^SSL^i ° f ^ MgW irti ^ e ^, Much in regard to the gene^XcSines wh IZ^ZZZ^^ ^ pamon of different plans of organisation hi, t» T . the COm ~ this difference between functiZflnZT; , Y du ° *° a ne S lect of between analogy and jff ^f^ correspondence, that is, tially dissimilar^ have he2 tlS 7 P benome * a ^ a re essen^ some who have clearly reL. Zf ^ ^ Same cate g 0I 7- But by reasoning, it has WaJSS^nT^.i!^^ aS the basis of ^ ^fc £* an unlimited ^ wThSfo ^ ^ ? ?** 0/ ^ same elementary parts exist S +?' haying been maintained that the Kingdoms, bJ^^^^^.^ Vegetable and Animal lies solely in the resn^n+iLT T between the several classes of each Weyerf ZZfyT ^2^""* ^ ^ ^ a d ° ctrine > sanction to it • as the m S °^ 7 aSSertl0n ' Smce Nature affords no -mat once ma?e obXus 7 "^ ° f ^ two of her k^S*™ f ^er^^^ the various tribes of to a certain 'archetype' or idZ LI? fandtbat ^y may all be referred stem with its foliacels and W ' COn T tuig of an tending axis or or root with its "bsorben^ fibres & ^ "°? ° f a descend -g a - generally attributable to th. S • m ° St ° bvious diversities are these component „art! • rt defici f c y or excess of some or other of massiye perfection ^ V f^ 7 .° f the treeS most remarkable for the flowers uS;:l7 ed ° f St man? £X ^ ^ ^^ P arts of their Wuty and loxo^l^STbl^ ££££?%^ «* «» But amid this general conformity the Bote W v^ a . true woody stem, tmct though subordinate typTs, ekch of them W? T** , W ° ^ dis " gradational forms, from the lofty tee ^ the ^ ^f^ a lon S se ries of between which consists rather i n ^ the divers^ ,?/ ai ? t; the diff erence very same elementary parts are ootII J J f tbe ^^ on wbi °h the superior elevation posted Conet^t^ ""S^, than in any the Palm and the Oak. w JI ?l°! er the °* ber - Tb ^ ^ -e compare the Palm and ^^0^^^?^ Thus if we compZ may be co ^ d ered as presenting typical ^ UK 7 P Cvi ^. i//*K»'- ■ ^ ) ? '"' C^/7tJL A /t-^t i , ^ fj l i y ■ 16 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. A 3 vJr - stem being exogenous and the embryo polycotyledonous. - examples of the Endogenous and Exogenous stems, we find that the same materials — cellular tissue, woody fibre, and ducts of various kinds — are worked-up, as it were, on two different patterns ; and as a like difference of plan extends itself also to the arrangement of the elementary parts of the leaf and to the number of the components of the flower, and even shows itself in almost the earliest stage of the life of the embryo, it becomes apparent that the diversity is one which belongs to the funda- mental nature of the two groups. There are instances, it is true, in which there is such a general conformity in external appearance between certain of their members (between Cycads and Palms for example), as might deceive a mere superficial observer \ yet there is no assumption of the essential characters of the latter of these groups by the former, the I So, again, although there are certain Flowering-plants (such as Lemna, duckweed, and Zostera, sea- wrack) which, alike in habit and in general simplicity of structure, correspond with aquatic Cryptogamia, these are at once reco- gnized as degraded forms of the Phanerogamic type, when their generative apparatus is examined ; reduced, though this is, to a condition of extreme simplicity. — The Cryptogamic series cannot be referred with equal pro- priety to a single c archetype,' so diversified are the types of structure, as well as of grades of development which its principal groups present. Still the plan on which their generative apparatus is constructed, though not so dissimilar to that of Phanerogamia as was formerly supposed (since no reasonable doubt can now remain of their true sexuality) present a certain fundamental uniformity ; whilst its several modifications serve to distinguish the subordinate groups of Ferns, Mosses, Liverworts, &c. Although the Cryptogamia as a whole rank below Flowering- plants, yet no one can help recognizing in a Tree-Fern a far more elaborate structure than that of Lemna or Zostera : so that the essential distinction between the two series lies, not in grade of development, but in type of conforma- tion. So among the Cryptogamia themselves, we find parallel series, such as those of Algce, Lichens, and Fungi, through each of which a cer- tain distinctive type is preserved, notwithstanding that between their several varieties of grade there is a close correspondence. — Hence we see that although, from the comparatively small number of distinct organs which the Plant possesses, and from the less complete separation even of these, there is not by any means the same scope for varieties in plan of organisation as we shall find in the Animal Kingdom, it is not the less certain that a considerable number of distinct types of structure exists, which cannot be reconciled to any other theory of fundamental unity, than that which refers them all to their common starting-point — the single cell. 17. Turning, now, to the Animal Kingdom, we find that even a slight general survey affords ground for the recognition of those four very dis- tinct plans of structure, which Cuvier was the first to mark-out clearly, namely, the Radiated, the Molluscous, the Articulated, and the Yerte- brated ; and these are found to be more and more clearly distinguishable from each other, the more profoundly we examine into the fundamental peculiarities of each, and the more fully we become acquainted with the history of its development. For by accurately studying and comparing the various modifications under which these respectively present them- '-**f*» ** * DIVERSITY OF TYPES OF ORGANISATION. 17 selves, we see that beneath the apparent mixture of characters which occasionally presents itself (as, for example, in the case of the Cirrhipeds, i 4), there is an essential conformity to one type, and that the departure trom the ordinary aspect is merely superficial, being such as adapts the animal or group of animals to a particular mode of existence. Now since modifications of a similar kind may take place in groups of animals belonging to different types, they may come to present very striking resemblances to each other in their adaptive characters (as is the case between Birds and Insects), although there is no conformity whatever in their general plan of structure. Taken as a whole, no animal belonging to any one of these types can be likened to any animal belonging to another ; although comparisons may be legitimately made between their individual organs. Thus, as Von Baer justly observes, « metamorphose a Cephalopod as you will, there is no making a Fish out of it, save bv bmldmg-up all the parts afresh •" yet in many portions of their organisa- tion Cephalopoda are unquestionably intermediate between the lower Mollusks and the typical Fishes. Again, although the higher Cepha- lopoda indubitably take a more elevated rank as Animals than the lowest ^^^.^^ as _ approximating more Man 7" "7. w ™ 7 W J1Sfl bears iar more resemblance to ^^ Cephalopod-Moreover it is to be observeu, ^ ,ne general type of construction manifests itself not merely in the mode in which the organs are grouped together, but also in the conformation of the organs themselves. Thus we shall hereafter see, that whilst there is L3T • + f rr f P° ndence betwee * *be condition of the Circulating Molluscous mESS^ ^ re i a I? e im P erfectio11 of *to oscular system, and its com- SX fr vl l V T r f]. cavit ^-^ere is a type which is peculiar to ^Th-T^^?^ 111 * he struc ^e of the heart, as well as in the AhT\ d + 1S J nbutl0 f of f e Wood-vessels. For the type of the heart, in the h a f fl . + + amm f ' - 1S ?* e J ongated dorsal vessel, which, if divided at all, ^ZT^ZlT^l "** f ° r *» —ral segments of the body j Mollusk nq 11Q iu T i, „ • x1 . , ~~.„,„v,v* vxgcu, «i™ muuu uiucKer wans, visually having the auricle or receiving cavity separated from the ven- similar ^ . ,-, * ° . •/ 7 r^^^^^s ^^ vuiLVJ. jlvj^vijujivu. vi similar parts than the occasional doubling of the auricle, where the two sets of P"l Si /^rhnnnA 4-\x ^ "Ul 1 j i it t .x - . _ returns ^o again, m the various Glandular organs of the Articulata, the required extent of surface is usually afforded by the elongation of a small number oi narrow tubes j whilst in the Mollusca, the same extension is provided tor by the multiplication of short and wide follicles. Yet we find that in *ttJi£?ZX%%Z !^ d in , TH »P«* t° the coitions of Mollusco mation is seen in the general olar, ofTw^ ^ ' [ ?*• ^Pe- culated in the Cn,.*^^^^^ ^ is as obviously arti- ^^%f AUm ^v iS ln type > ° r P lan of organisation, that the most it k ^ .^f*** 1^ among the several form! of Plants and Animalt it s not the less true that they are extinguished by very marked dTvt sities m arade of dewlnnme™ / . i™ ,„n -i -A _r_ / , J , , Kea ai ver- yf develop c ' \ 18 GENEBAL PLAN OF ORGANIC STKUCTUBE AND DEVELOPMENT. which the several parts that make up the entire fabric are characterised by specialities of conformation, so that each becomes a distinct organ, adapted to perform a function more or less different from that which other parts can discharge. The lower we descend in the scale of being; whether in the Animal or in the Vegetable series, the nearer approach do we make to that homogeneousness which is the typical attribute of inorganic bodies, wherein every particle has all the characters of indivi- duality, so that there is no distinction either of tissues or of organs. Thus in Sponges and Sea-weeds, even when of considerable size, every part resembles other parts in intimate structure, and differs but slightly from them even in external configuration ; so that the whole mass is little else than a repetition of the same organic components. On the other hand, as we ascend the scale of being, we find the fabric — whether of the Plant or the Animal — becoming more and more heterogeneous; that is, to use Von Baer's language, " a differentiation of the body into organic systems, and of these again into separate more individualized sections/' presents itself. Thus, as we ascend from the lowest towards the highest forms of Vege- table life, we find that out of the homogeneous aggregation of cells which forms the simple frond of the humble Algse (§ 22), a differentiation gradually arises between the < axis' and the < appendages to the axis f that m the axis, there is a gradual separation established between the ascend- ing portion, or stem, and the descending portion, or root ; and that among its appendages, the foliaceous organs become more and more completely separated from the generative apparatus. Even in the highest Plants however, we find an extensive repetition of similar parts; and there is always, too, a close correspondence in the intimate structure of even the most antagonistic organs, such as the roots and leaves.— The differentia- tion, both as regards external conformation and intimate structure, pro- ceeds to a far wider extent in the Animal kingdom, in virtue of the much greater variety of purposes to be attained in its existence ; and we see this carried to its highest degree in Man, in whose organism the principle of specialisation everywhere manifests itself, no part being a precise repe- tition of any other, except of the corresponding part on the opposite side of the body.* 19 It is only, however, by a very gradual succession of steps, that this elevation is attained. The simplest Animals are precisely upon a level with the simplest Plants, as regards their homogeneity of character • and no sooner does a differentiation of organs show itself, than these are in the first instance almost indefinitely repeated, so that, however numerous may be the parts of which the entire organism is composed, they are (so the/? Mollusca Zoophyt ?/ the rest, can maintain an independent existence, and are therefore com- monly accounted distinct individuals. But we find the same to hold good, as * This fact is most curiously exemplified in the speciality of the seats of election of lo°I- m l\ I i Q utl ^ 10n ' + , w c ° bvi0USly depend u P° n the P resen <* of * series 7* ™ the blood rather than upon any primary local disturbance.— See Dr. William •Budds Memoir on "Symmetrical Diseases," in the " Medico-Chirurgical Transactions," ^Autwl «p" f 6 S n pi" .^W, 1 I^thology," Vol. I. p. 17 etseq., and tue Authors "Principles of Human Physiology," 4th Ed. p. 192. ii IE : REPETITION OF SIMILAR PARTS IN LOWER ORGANISMS. 19 d f s s Fig. 8. regards individual organs, in the highest members of each of the Inverte- lated sub-kingdoms, and even (though to a less extent) among Verte- ren + v am T 1S ' ThuS amon 8 tlie E ' chinodermata, there is a precise v Jr!-i-° U • s 1 imilar P arts around a common centre : and although this repetition is limited to b five in the highest forms of the class, yet it extends to a much greater num- ber in those of inferior organisation, — as we see m comparing the Ophiura with its five simple arms (Fig. 8) and the Penta- cnnus (Fig. 9), whose ten arms all subdivide into such numerous branches, that the aggregate num- ber of pieces in the whole is estimated at above a hundred thousand. So, again, in the Cephalopo- da, which constitute the highest division of the Molluscous series, we find the tentacula sur- M Ophiura. TttaVathZTlv - ^ .t m0S v ^^ly Spiled m the lower reduoir+T • Tf x 1S '° n (^ awt ^ us and its allies) ; whilst the V are reduced tonight or ten in the dibranchiat? «~J- trujU,. ^ l-L are the same time acquiring a much higher individual development, and often hav- fg one pair differentiated from the rest tor some special purpose. So in the -fish Fig. 9. tne Articulate series, we find the loco- motive, respiratory, and other important organs almost indefinitely multiplied in tne longitudinally - repeated segments; out as we ascend towards the higher Articulata, the number of segments becomes strictly limited and greatly reduced, even where these divisions are still httle else than repetitions of one another,_being only twenty-two in the Centipede, and thirteen in the Insect Larva ; whilst in the perfect Insect, the differentiation is carried to its bi«W extent tlie locomotive apparatus oemg restricted to the three thoracic segments and all the other organs, even when re- peated throughout, being unequally de- Pentacrinites brkireus Sloped „ th0 ^ ^ o, ^ pr . ncip]e ^ grafcai ^ c2 { 20 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. shows itself most remarkably in the conformation of the members of Verte- brata: for, taking the many-jointed but j single rod-like appendage of the Lepidosiren (Fig. 3, a, and Fig. 1 50) as their lowest type, we find this simply repeated even to the extent of a hundred-fold or more, in the digital rays supporting each of the pectoral and ventral fins of Fishes ; as we ascend thence, through the extinct Enaliosauria (Ichthyosaurus, Plesiosaurus, &c.) to the typical Reptiles, we find the number of these multiplied digits dimi- nishing, until it settles down at five, and the number of joints in each also reduced, until it becomes restricted to the six rows (two carpal, one metacarpal, and three phalangeal) which characterise the hand (or foot) of Man ; in Birds and Flying Mammals there is a most marked differen- tiation between the anterior and posterior extremities, as there is also (though in a less degree) in Man ; and in the Quadrumana, we begin to see that specialisation of the first digit (this being usually common to all their members) which is carried to its highest point in the hand of Man, whose other digits, also, have their distinctive capabilities, whereby this member as a whole becomes the most highly-organised of all instruments, in virtue of the unequalled variety of actions which it is adapted to perform. 20. Thus we see that, whether we trace the ' Archetype' of each great subdivision of the Animal kingdom into those modifications which it presents in the more restricted groups, — or whether we follow any organ or system, from the form under which it at first presents itself, to that which it assumes in its state of most complete development, — we recognise one and the same plan of progression, namely, from the general to the special; and, as Yon Baer justly remarks, the relations of any organised fabric to any other, must be expressed by the product of its type with its if developme Neither alone suffices to characterise it ; for under the same type, different grades of development may present themselves ; whilst conversely, a like grade of development may be attained under different types. And this general fact needs to be constantly borne in mind, nbt merely when a Plant or Animal is being considered as a whole, but also when we are studying the evolution of any individual organ or system in the ascending series ; since it is no more possible to follow this through one unbroken progression, than it is to arrange the entire assem- blage of beings composing either kingdom in a single linear series. — It may in some degree assist the reader in his perusal of the subsequent pages, if we here pause to take a general survey of the principal types of Vegetable and Animal conformation, and of the chief diversities in grade of development which present themselves under each. 21. Vegetable Kingdom. — If we commence by examining any Plant of high organization, we observe, in the first place, that there is a complete differentiation between its organs of Nutrition and its nrtmns nf 7sWv>- duction; and further, that its principal organs of Nutrition, the root and the leaf are separated from each other by the interposition of the stem or axis, around which the various appendages are arranged with a con- siderable degree of regularity. Further, we notice that a corresponding differentiation presents itself, as to the intimate structure of these several organs; for whilst the parts most directly concerned in the vital opera- GENERAL VIEW OF VEGETABLE KINGDOM. PROTOPHYTA. tions of the organism are chiefly made-up of aggregations of cells, which resemble in all essential particulars those of which the simpler forms of vegetation entirely consist, these are supported upon a framework of woody fibre, an extension of that which J stem and roots ; and further, in order that air and liquids may the more readily find their way from one part of the structure to another, than they could do by transmission from cell to cell, a set of ducts is inter- posed, which establish a ready communication through the stem between the roots and the leaves. These organs are all mutually dependent and connected ; and contribute, each in its own special manner, to the life of the Plant as a whole. But since all the most essential organs are many times repeated, the loss of some of these does not involve the destruction of the entire organism; and even the separated parts may develope the organs in which they are deficient, and may thus evolve themselves into entire plants, and maintain an independent existence.' 55 ' In this way a multi- plication of the products of the original germ may be effected ; but these, as will be shown hereafter (chap, xi.), are not distinct individuals in the highest sense of that term ; and the process by which they are evolved is simply a modification of the ordinary Nutritive operation, and is so far from being a form of true Generation, as to be essentially antagonistic to it. This distinction is one of much importance ; since on it depends the recognition of the organs in Cryptogamia, which are homologous with those of Flowering-Plants. 22. Having thus determined, by the analysis of one of the highest Plants, what it is that constitutes the most complete type of Vegetable organisation, we shall commence with the lowest division of the series, and endeavour to trace-out the principal lines of ascent by which that type is attained. This can only be accomplished, at present, in a very imperfect manner ; since it is only within a very recent period, that the homologies of the reproductive apparatus of Phanerogamia have been discovered among Cryptogamia ; and little more than a guess can be as yet made, as to the conditions which these present in some of the humbler forms of Cryptogamic life. — The lowest type of vegetable existence is afforded by those organisms, which either consist of single cells, or of •>/ fi other cells, save for the purpose of generation, of which the re-union of the contents of two cells, by an act of ' conjugation,' is an essential condi- tion. Any one of these cells may multiply itself indefinitely by sub- division, the results of which process are seen in the accompanying ex- ample (Fig. 10) ) but these products are all mere repetitions one of another, and often detach themselves spontaneously, so that the descendants of a single cell may cover a very extended area, as is the case, for example, * This is usually the case under favourable conditions with regard to leaf -buds, which can put forth rootlets, and then evolve a stem, from which other leaf-buds and their flower-buds are developed. But there are some plants, as Bryophyllum, which have the same power in every leaf, or even in every fragment of a leaf ; a small portion, laid upon damp earth, or suspended in a humid atmosphere, gradually evolving itself into the entire organ, and at the same time developing the other parts most essential to the performance of its nutritive operations, from which the reproductive apparatus is subsequently put forth. * 1 1 22 PLAN Fig. 10. with the Protococcus nivalis, or ' red snow.' There is here, therefore, not the least show of differentiation ; no special cells being set apart even for the performance of the generative act. Where the multiplied cells remain in continuous connexion with each other, being imbedded in a common sub- stratum of gelatinous substance, so as to form but a single mass (Fig. 10), this may be perfectly homoge- neous throughout ; no definite form being presented by it as a whole, and no trace of < organs' being dis- tinguishable in any part of it. The first indication of progress towards a higher grade, is given by the limitation of the direction in which the increase takes place : so that, instead of an amorphous aggre- gation of cells, we find a linear series (Fig. 11, a) which is formed Iby successive trans- verse subdivision ; and this filament may increase in breadth by longitu- dinal subdivision(B), Hormospora transver satis. Pig. 11. so as at last to pro- duce a laminar ex- pansion, such as that of the common Ulvce, which is termed thallus. In a the simplest forms of this thallus, we do not meet with the slightest trace of differentiation; and . appears to live as much for and by itself, as if it were completely detached from the rest. Bangia velutina. every one of its component cells Every one of them, moreover, seems able to multiply itself, not merely by sub- division but also by the emission of a portion of its contents enclosed m a cell-wall, m the condition of a 'spore' or detached gemma; and this in the tribe now under consideration being usually furnished with cilia, and endowed with the power of spontaneously moving for a time, is termed a 'zoospore.' When the zoospore has been thus carried to a distance from the organism from which it proceeded, it begins to develope itself into a similar organism by the process of duplicative subdivision ; and in arriving at the highest of these stages of development it passes through the simpler forms which remain permanent in vet humbler grades of vegetation. The true Generation of the plants of this group, to which the term Protophytes may perhaps be advantageously restricted, seems to be always accomplished by the process of ' conjugation ' m which any or all of the component cells may alike participate : but we see in its higher forms, a tendency to the distinction between the ' sperm- cell and the ' germ-cell,' that is, to the differentiation of sexes into male and female,— the only mark of heterogeneousness which yet presents itself. I he product of this act is a new cell, from which a new plant originates GENERAL VIEW OF VEGETABLE KINGDOM. ALG.E. 23 t b y duplicative subdivision, as in the case of the zoospore. Here, then, we find that each individual (understanding by this term the aggregate result of a generative act) is made up of an indefinite number of cells, winch, being precisely similar to each other, have no relation of mutual dependence ; so that the Life of the whole is merely the sum of the lives ot the component parts, and not, as in higher organisms, the product of it. 23. In the next stage of development, the differentiation of parts begins to manifest itself more decidedly ; but this not so much in a distinction ot organs adapted to separate offices in the act of Nutrition, as in the limitation of the Reproductive act to particular portions of the organism, and m the setting-apart of special organs for its performance. For we nave as yet no real distinction between stem, roots, and leaves ; although some semblance of such a distinction may present itself. The primordial cell, by repeated subdivision, extends itself into a < thallus,' whose form has but little definiteness, and whose tissue is nearly homogeneous throughout, being entirely composed of cells of various forms, without either woody fibres or vessels of any kind; and it is chiefly by its appa- ratus of fructification which presents itself under many different aspects, that this group, which may be designated by the term Thallogens is distinguished m from the preceding. Nearly the same degree of general development is presented by three tribes of these humble Cryptogamia, —namely, Alga* Lichens, and Fungi,— which, nevertheless, are fitted to exist under very diverse conditions, and which present corresponding diversities of structural type ; and all of them seem to agree (according to the most recent investigations, of which an account will be given here- ?\°5; A1E \? I V m the P ossession of a special generative apparatus, in which the distinction of sexes is clearly marked. This consists of a set 01 sperm- cells developed in certain parts of the organism, and of a set of germ-cells evolved elsewhere, usually (but not always) in the same indi- vidual ; the product of the former is a < spermatoid' body, which comes into contact with the latter and fertilizes its contents ; and the result is the formation of a germ, which must be considered as the commencement ot a new generation. This germ, however, frequently remains for some time m connection with the parent, and multiplies itself by duplicative subdivision at the expense of the nutriment which it draws from it, so as at last to evolve itself into a collection of < spores' contained within a special envelope, every one of which, when liberated from, the parent, may develope itself into a new plant in which the same processes are repeated. It is by the general relation of this apparatus of fructification to that of nutrition, that the three groups already named are most dis- tinctively characterized. 24. Thus the Algce vegetate exclusively in water or in damp situa- tions ; they require no nutriment but such as is supplied by water and by the air and inorganic substances dissolved in it ; they absorb this nutriment equally by every part of their surface ; and they show a great tendency to the extension of the < thallus' by the multiplication of cells m continuity with the existing fabric, so that it frequently attains most * The group of Algse as here limited, does not include the Protophytes described in the preceding paragraph; for although these, being mostly aquatic plants, are usually ranked m it, yet their type of reproductive apparatus is so distinct from that of the hidier A 1 *» as to require that they should be separately considered. b ' * ! 24 ORGANIC we find CT ^T + n T S - IU S ° me ° f the sim P ler forms of the group, Lis tf whiowf f IT" 1106 T? th Z Se negations of similarly-shaped Mesolil)vi 1 ^ fa t r r ? the ^^Phyta are made up. W in 3L% 2 f 2) ' althou ^ we *»™ a d^tinct axis with radiating rent wh! 1 e ' tt 1 T^ ™ °T 5°^ ° f el0ngated Cells Ve ^ loosel 7 "^ rent while the latter consist of single rows, bearing the generative cells the fa uctification that raises it above the type of an Ulva. In the highest Fig. 12. UtUlh Fig. 13. / if ^\1* n ^-- .c=^ ffi ^ vl.Vv &; ^ X -/ ^ ^ (SS> !>'• & -«<•# 1 • ** Frond of Osmunda regalis ; — a, sterile or folia- >ous portion ; b, fertile portion :— a, part of the ceous x y latter enlarged, to show the ihecse. JEquisetum arvense "^ GENERAL VIEW OF THE VEGETABLE KINGDOM. FERNS. 31 * but a proper descending axis or true root, from which alone the radical fibres are given off. In the stem, the cortical portion is separated from the medullary by the interposition of bundles composed of woody fibre and vascular tissue; and the principal difference which exists between these and the woody layers of Exogenous stems, lies in the absence of >gul From the fibro- vascular bundles in the stem, prolongations are given off, which pass into the leaf-stalks, and thence into the mid-rib and lateral branches of the foliaceous appendages, to which they form a kind of skeleton, as in the leaves of Phanerogamia. These organs, which are distinguished as ' fronds,' on account of their combining the character of a leaf with that of an apparatus of fructification, are constructed upon the same type with the leaves of Flowering-Plants; being composed of a cellular parenchyma, enclosed between two layers of epidermis, and having air-chambers to which access is given by stomata; and they can scarcely be less com- plete as organs of nutrition, although still made to bear a share in the function of reproduction. Even in this respect, however, a differentiation exhibits itself in certain Ferns, as the Osmunda regalis (Fig. 28); whose fructification is restricted to particular fronds, or parts of fronds, hence designated ' fertile,' which lose their foliaceous character; whilst the remainder bear no fructification, and are hence designated as * sterile,' performing the functions of leaves alone. The ostensible organs of fructi- fication are far from constituting (as they were until lately supposed to do) the real generative apparatus; for this is evolved at a period in the life of the plant, at which its appearance was totally unexpected. Each of the ' spore-cells' which are set free from conceptacles on the under surface of the fronds (Fig. 27), when received upon a damp soil, extends itself, by duplicative subdivision, into a frondose body closely the thallus of the Marchantia: it is in this that the 'snerm- Fig. 30 Wife <% I I i ' Lyeopodium eemuum. ■ ■ 32 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. "Marsilea quadrifolia. cells' and c germ-cells' are evolved, and that the fertilization of the latter, by self-moving spermatoid filaro ents set free from Fig. 31. the former, takes place ; and from the embryo- cell, which is the product of this operation, there arises — not, as in the Mosses and Liverworts, a conceptacle filled with spores, each of which may give origin to a separate plant, — but a single young Fern, which, having attained its full deve- lopment by duplicative subdivision, detaches cer- tain of its cells, as c spores,' to continue the race by the same process. In this departure from the plan which prevails among the inferior Crypto- gamia, we have an obvious tendency towards that of the Flowering-Plants : the entire product of each generative act being worked-up (so to speak) in the Fern, as in the Flowering Plant, into the diversified parts of a single organism ; instead of being subdivided, as in the inferior Cryptogamia, amongst an indefinite number of independent fabrics, which are mere repetitions one of another. Still the type of the generative apparatus in the Ferns is essentially Cryptogamic. — That of the Equisetacece (Fig. 29) appears to be essentially the same; but in Lycopodiacece (Fig. 30), Isoe- tacece, and RhizocarpecB (Fig. 31), there is a still closer approximation to the Phanerogamic type, the ' sperm-cells' (' small spores') being directly produced by the parent-structure, and the ' germ-cells' alone being evolved, after the detachment of the i large spores,' upon the ' prothallium' into which each of these developes itself. 29. The distinctive character of the Phanerogamia or ' Flowering- Plants' is not the possession of what are commonly designated as ' flowers,' since these may be reduced to a condition in which they are scarcely distinguishable from the fructification of the Cryptogamia. In fact, the group of Rhizocarpeae, in which the concurrent action of the small and large spores had been ascertained to be necessary for the production of an embryo, was referred by many Botanists to this division, at a period when the existence of distinct sexes had not been recognized among the Cryptogamia generally, and when it was, in fact, not merely doubted, but usually denied. Still, it is in the peculiar type of their Generative apparatus, that the essential distinction lies ; for the fertilizing process is performed among them in a manner not elsewhere seen, namely, by the emission of a long tube from the ' germ-cell' (pollen-grain), which finds its way (often through a distance of some inches) to the ' sperm-cell' buried in the ovule ; and it is among them alone that a true seed is pro- duced, in which, with the embryo, a store of ready-prepared nutriment is laid-up for its early development. This sub-division of the Vegetable kingdom includes a vast range of species that differ very greatly in the degree of development, both of their nutritive and their generative appa- ratus; but for our present purpose, it will be sufficient to sketch the typical plan, which is more or less obviously manifested in the conforma- tion of the entire group. — If we analyse the fabric of any common Phanero- gamous Plant, we find that it consists essentially of an axis and appendages; * ™^ ~t GENERAL VIEW OF VEGETABLE KINGDOM. PHANEROGAMIA. 33 and of a the former being made up of an ascending portion or stem, descending portion or root, with their respective ramifications ; and the latter being distinguishable foliaceous Aoral Fig. 32. 15 C organs, which will be pre- sently shown to be modifi- cations of the same funda- mental parts. The axis (Fig. 32, a, a a) is composed of cellular parenchyma, with a larger or smaller proportion of fibro-vascular tissue ; and it is upon the mode in which these com- ponents are arranged rela- tively to each other, and in which progressive additions are made to the diameter of the axis, that the dis- tinction is founded between the Endogenous and Exo- genous types, which, toge- ther with corresponding distinctions in the struc- ture of the leaves, flowers, and seeds, affords a basis for the sub-division of the Phanerogamia into two pri- mary classes. From the central axis, bundles of fibro-vascular tissue pass down into the root-fibres which form the ultimate ramifications of its descend- ing portion; these are en- veloped in firm tissue, that limits their absorbent power to their extremities, which, being still soft and succulent, are known as < spongioles.' On the other hand, the fibro-vascular bundles of the ascending portion of the axis pass into the footstalks of the leaves; and their ultimate ramifications form the skeletons of these organs, the interstices being filled up with cellular parenchyma, and the whole being clothed with an epidermis, quite distinct in texture from the parenchyma it covers, and perforated by the peculiar apertures termed 'stomata' (Fig. 155). Various modifications present themselves in the form of the leaves, and in the arrangement of their component parts ; but none of these affect the essential character of the organs. The modes, too, in which they are arranged on the stem, present a great apparent variety ; but they seem all reducible to one fundamental type, namely, a spiral, which is the result of the radiation of the appendages, not from a single a, Ideal Plant, after Schleiden ; a to avi, the axis, a being the root, ai, aii, a*' 1 , aiv, and «v the successive mternodes of the stem, and avi the terminal development of the axis into an ovule ; b, rootlets ; c to cvii the successive foliaceous ap- pendages to the axis, c being the cotyledons, ci, en, and cm the ordinary leaves, civ the outer floral leaves or sepals cv the inner floral leaves or petals, cvi the stamens, and cvn tne carpellary leaves; d, leaf-buds :— B, carpel enclosing an ovule, seen externally and in section, showing a the stigma b the style, c the ovary :-c, leaf-buds, as seen externally at d' 9 and in section at d". \?* 34 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. point, but from a longitudinal axis. When am a *v at - ._* esiilar in a vertical line one with another, — the second not being above the first, but a little to one side of it, — the third holding the same relation to the second, — and so on ; in such a manner that a line carried through the points of origin of the successive leaves, which are termed ' nodes,' will not only ascend the stem, but will gradually turn round it, and will at through The leaves whose origin has been intersected by this line, whilst it makes one turn round the stem, are said to form a cycle; and the number of leaves ations. Thus in Dicoty- great will b< so on. hefl Monocotyledo fourth leaf being above the first, the seventh above the fourth, and so on. There are cases in which the cycle seems to consist of only two leaves; each leaf springing from the side of the stem precisely opposite to that from which the leaf below it, as well as the one above it, arises. The most common departures from the spiral type, shown in the disposition of the leaves, are those which are known as the opposite and the verticillate (or radiate) arrangements. three modes, each of which has some evidence to recommend it ; and perhaps the deviation does not always take place in the same way. * The complete floral apparatus of Phanerogamia consists externally perianth,' composed of a series of verticils of foliaceous organs, which do not depart widely, except in colour, from the ordinary type of the leaf, and are arranged according to the law of spiral development round the axis. *" " of a >gul For the first or outermost layer of the ' perianth,' in a posed of a whorl of sepals (Fig. 32, c iv ) alternating with the preceding; and the corolla, in like manner, consists of a whorl of petals (c v ), which alternates with that of the sepals, but corresponds with that of the bracts. These whorls, in many flowers, are considerably multiplied, and the spiral arrangement of their component parts is often very obvious ; and, when * Thus, ' opposite' leaves would be produced in a plant whose * cycle' consisted only of two, by the non-development of every alternate segment, or < internode' of the stem, so that each leaf and its successor on the opposite side come to be developed from the same part of the stem,^ whilst separated by an interval from the next pair. But this explanation does not suit those cases, in which the successive pairs of leaves are arranged on the stem at right angles to each other ; and this arrangement may either be attributed to the development of two opposite leaves from each node, the successive pairs being then arranged in a cycle of four; or to the existence of two spirals proceeding up the stem simultaneously. — In like manner, a ' verticil' of five leaves originating from the same point of the stem, may be conceived to result from the non-development of the internodes between five successive nodes ; and it sometimes happens that leaves which have a ver- ticillate arrangement at one part of the stem, are spiral at another, being separated by the development of the intermediate internodes. But this does not account for the fact, that the successive whorls themselves usually alternate with each other ; each leaf of the verticil being over the spaces between the leaves of the verticil beneath it. —And here again it would seem necessary, either to imagine that all the leaves of one verticil may originate from a single internode, or to suppose several spirals to be passing round the stem. In either way, however, this very common arrangement is reconcilable with the general theory of spiral development, which is thus readilv carried into application as regards the disposition of the parts of the Flower. ■ * I li GENERAL VIEW OF VEGETABLE KINGDOM. PHANEROGAMIA. 35 such is the case (as in the Garden Pseony), we may observe such a gradual passage from the type of the ordinary leaf, through the succession of bracts and sepals, to the most characteristic petal, that the essential con- formity of this last to the same general type with the preceding cannot be for a moment doubted. In the flowers of Dicotyledons, the typical number of components of each whorl, as of that of the cycle of ordinary Jive, whilst in the Monocotyledons The regularity of a flower may be interfered-with by the suppression or by the multiplica- tion of whorls ; but the greatest departures from archetypal simplicity are those which result from the unequal development of different parts of the same whorl, some being very imperfectly evolved or even entirely suppressed, whilst others are extraordinarily augmented in size, and strangely altered in figure and character. The scientific Botanist, how- ever, can seldom be at a loss in the investigation of their real nature, if he proceed on the morphological principles already explained ; and h e continually finds his determinations justified by the occurrence of ' mon- strosities ' which exhibit a more or less complete reversion to the arche- typal form (§ The non-essential character of the perianth is indi- cated by the deficiency of one or more of its whorls in many tribes of Plants, which are nevertheless truly Phanerogamic. It is interesting to remark, however, that the group of Gymnospermce, in which the deficiency is most complete, really form a transition-step to the higher Cryptogamia, in virtue of certain peculiarities in their proper generative apparatus, which will be explained hereafter (chap. xi). — It is within the protection of the perianth, that the true generative organs are developed ; and these consist of the anthers, (Fig. 32, c vi ) from which the i sperm-cells' (here termed pollen-grains) are evolved, and the carpels (c vii ), whose aggrega- tion forms the pistil, containing the ovules (a vi ), each of which includes a ' germ-cell' imbedded in a mass of nutritious matter, the whole invested by two or more seed-coats. Now the anthers, which with their support- ing ' filaments' constitute the stamens, depart more widely than do the sepals and petals from the ordinary condition of the leaf; but it is quite certain, alike from the history of their development, from the series of intermediate forms which some flowers (as the Nymphcea alba, or white water-lily) present, and from their occasional reversion in monstrous flowers to the form of petal or sepal, or even to that of the ordinary leaf, that they too belong to the same type of structure. The carpels (b), again, may be regarded as leaves folded together at the edges ; as is indicated by their frequent retention of much of the leafy character, even in the normally-developed flower, and by their occasional more or less complete reversion to the type of the leaf in monstrous blossoms, sometimes when (as in the common ' double cherry') the stamens have undergone a less complete transformation. In the Gymnosperms, indeed, the carpellary leaves are not folded together so as to enclose the ovules, which are deve- loped upon their internal surfaces ; and merely protect them during their immaturity, by their own mutual adhesion. — It is the general rule for the two kinds of sexual organs to be developed in the same organism ; and where, as is most commonly the case, every flower contains both stamens and carpels, it is said to be hermaphrodite. There are certain cases, however, in which, by the suppression of one or^ other of these whorls, the flowers become unisexual; when the staminiferous or male b2 I i * I i t 36 AND flowers are borne on the same plant or tree with the pistilline, it is said to be monoecious ; whilst if the two sets of flowers are developed by dif- This last arrange- ferent individuals, the species is said to be dioecious. ment, in which the generative apparatus attains its highest degree & of differentiation, is comparatively infrequent j but we find examples of it in several groups of Cryptogamia, as well as among Phanerogamia. 3 1 . The ' embryo-cell,' which is formed within the germ-cell, after the own already adverted-to, developes itself by duplicative subdivision, just as requires tor the continuance of this operation is furnished by the store previously ovule subdividing to constitute a multitude of independent organisms, remains connected so as to form but a single fabric ; and this exhibits at a very early period a tendency to become heterogeneous, by the development of distinct organs, every kind of organ, however, being very numerously repeated. For, at the time that the seed is detached, as a self-sustaining struc- ture, from the parent, the embryonic rudiments of the stem and root are already formed, and a temporary leaf-like expansion, the single or double cotyledon (Fig. 32, a, c), is prepared to evolve itself; whilst a supply of nutriment for its further development is stored-up within it, either form- ing a separate albumen external to the embryo, or being contained within its cotyledons, which are in that case thick and fleshy. The subsequent evolution of the plant, of which < germination' is the first stage, consists in the progressive development of the ascending and descending axes and of their respective ramifications, these remaining permanent; and in the evolution, from the ascending axis, of a succession of mutually-similar appendages, foliaceous and floral, which have only a temporary existence, each set being in its turn replaced by another. Thus the individuality of the whole fabric is maintained, whilst a continual change is taking place in certain of its component parts. 32. It is with the performance of the true generative act, and the consequent production of a new embryo-cell, that each " new generation" originates. But it is not in this mode alone, that Phanerogamic Plants (for the most part at least) are multiplied. For each leaf-bud usually possesses within itself the capacity of putting forth roots, when separated from the parent-stock and placed in circumstances favourable to its growth, so that it thus becomes capable of maintaining an independent existence, and of developing itself into a perfect Plant ; and there are some Phanerogamia which spontaneously detach leaf-buds or ' bulbels ' and which thus multiply themselves after a manner analogous to that which prevails so remarkably among the lower Cryptogamia. This is pre- eminently the case, for example, with the common Lemna (duck-weed) each plant of which consists of but a single foliaceous body, with a root- fibre hanging from its under surface; this puts forth buds from its margin ; and these buds, early detaching themselves from their stocks henceforth maintain an independent existence, so that the plant thus becomes rapidly multiplied by gemmation, large surfaces of water being covered by the growth proceeding from a single individual, without the intervention of any process of generation. —It is interesting to remark that this little plant seems to hold almost the same relation to GENERAL VIEW OF ANIMAL KINGDOM. 37 Phanerogamia, that the lowest Protophyta do to Cryptogamia. For it scarcely presents any distinction of parts, the leaf and stem being fused together into a single flattened lobe, whilst the organs of reproduction are reduced to their very simplest form, being developed in a slit in its edge. Its texture, too, is of the simplest kind, being composed of scarcely any- thing but ordinary cellular tissue. And the developmental process here, as in the Protococci, consists in the multiplication of organs which repeat each other in every particular, and which, having no relation of mutual dependence, can exist as well detached as coherent ; instead of tending, as in the higher forms of Vegetable life, to the evolution of a single fabric, whose several parts present a marked differentiation of external form and of internal structure, and have such a functional dependence on one another, that they can only exist as living bodies so long as they remain mutually connected. 33. Animal Kingdom. — Turning, now, to the other great division of the Organised Creation, we shall in the first place examine, as in the previous case, what is the highest form under which its life expresses itself. The whole nisus of Yegetative existence consists in the activity of the organs of Nutrition and Reproduction ; but, on the other hand, the nisus of Animal life tends towards the evolution of the faculties of Sensation and of Self-determined motion, and, in its highest manifestation, to that of the Intelligence and Will. The instruments of these faculties, how- ever, are in the first place developed, and are afterwards sustained, by the Organic apparatus with which they are connected ; whilst, in their turn, they become subservient to its operations : so that, in those forms of Animal existence, in which there is the greatest differentiation of organs, there is at the same time the closest relation of mutual depen- dence in their actions; and every thing tends to render the entire pro- duct of each generative act a single individual, in the most restricted sense of that term, no multiplication by the subdivision of that product ever taking place (save as a monstrosity), but the whole of it evolving itself into a congeries of different but mutually-related organs. It is only in the higher forms of Animal existence, however, that we meet with this complete individualisation, and this marked predominance of the animal over the vegetative. In a large proportion of the beings composing this kingdom, the apparatus which is subservient to the strictly animal functions is scarcely differentiated from that which ministers to organic life ; in many of the cases in which the former is separately distinguished, it seems but a mere appendage to the latter; and it is only in the highest or Vertebrate type, that we find the general plan of the fabric distinctly arranged with special reference to the mani- festations of Animal power, which involve the exercise of its highest attri- bute Intelligence. The nearest approach to this is made in the higher forms of the Articulated series ; in which a very remarkable degree of development is given to the instruments of the lower animal powers, especially the locomotive apparatus ; and in which the general plan of structure, and the arrangement of the nutritive apparatus, have evident reference to this. But in the Mollusca, we find a marked predominance of the Vegetative apparatus ; it being in only a small proportion of the group, that there is any considerable power of movement. And in the Radiata, it becomes obvious that the general plan has reference rather i ""■ I 38 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. to the ' vegetative repetition' of the organs of Nutrition and Reproduction, than to any manifestation of the higher Animal powers ; the apparatus for which, in so far as it is developed, exhibits a like repetition of similar parts.— Notwithstanding the diversity of these types of structure, how- ever, and the marked differences which they present in regard to the relative development of their several organs, we observe in the hio-her forms (at least) of each of them, a differentiation of all the most important parts by which the Animal is especially characterised. For we find in each type a digestive cavity for the reception and preparation of aliment ; chyliferous channels or vessels, into which the liquid prepared by the diges- tive process transudes from this cavity, to be conveyed to the remoter parts of the organism ; a circulating system, by which the distribution of the nutritive fluid is effected, the surplus materials brought back, and the waste or refuse matter removed from the tissues and conveyed for elimi- nation to appropriate organs ; a respiratory surface, through which the circulating fl aid is exposed to the influence of atmospheric air • secreting glands for the separation of certain products from the blood, either for its purification, or for special uses in the economy, or for both purposes combined ; generative orgcms, in which ' sperm-cells,' or ' germ-cells,' or both, are developed, the latter being enclosed (as in Phanerogamous Plants) in a store of nutriment prepared for the nutrition of the germ, so as to constitute an ovum; organs of support and protection, forming a < skeleton' of some kind, either external or internal; organs of sensatSm; organs of consciousness and self-direction ; and organs of locomotion. 34. It is true that in the least-developed forms of each type, we may find some or other of these organs but little distinguished from the ] structure, or even entirely absent ; but the proportion of such forms is smaller, the higher we ascend in the scale. Thus, in a large part of the Radiated series, there is but little differentiation of the several parts of the nutritive apparatus • and although the reproductive is nearl y always very distinct from it, yet even this is scarcely segregated in the lowest examples of the type : whilst even the very slight develop- ment which the organs of animal life attain in the higher Radiata, is altogether wanting in the lower, among which they are not distinguish- able by any structural mark.— But in the Molluscous series, it is only among the very lowest that we have a difficulty in distinguishing all the essential parts of the apparatus of nutrition and reproduction, the absorbent and circulating apparatus being usually that which is most imperfectly developed; and although the organs of sense and locomotion are not evolved in the same proportion, we never fail to find a nervous ganglion which must be considered as marking the existence of some degree of consciousness.— On the other hand, in the lowest forms of the Articu- lated series, it is the imperfection of the nutritive apparatus which most strikes us ; and although distinct sensori- ' - - . - deficient, yet they present themselves very- higher . - very prominently in ...q.,,.. parts of this series, in which the type of nutritive system is still com- paratively low. In both these sub-kingdoms, however, it is only in a small proportion of each series respectively, that we fail to discern all the essential parts of the assemblage of organs j ust now enumerated ; those higher forms of each, in which the differentiation is complete, constitut- ing the great bulk of its entire series, instead of being, as among the HI GENERAL VIEW OF ANIMAL KINGDOM. PROTOZOA. 39 Radiate, exceptional as to number, and probably to be so considered in regard to type likewise.* — Now, in the Vertebrated series, the complete differentiation of all these structures is nearly the invariable rule j it being only in one of the very lowest Fishes (the Amphioxus), that we meet with such an imperfect development of any of the systems above enumerated, as reminds us of those simpler organisms in which they are absolutely deficient.— There is another point of interest nearly related to the preceding, in regard to which these primary types of Animal confor- mation present a marked contrast ; and this is the degree in which they are severally capable of being multiplied by gemmation. This power exists among Zoophytes in exactly the same degree as among the higher Plants ; for, whilst the gemmae, in the former as in the latter, usually remain connected with the parent-stock, they are capable of maintaining their existence if detached, and are regularly thrown-off in some species, so a s to become independent organisms, possessing all the capabilities of that from which they have separated themselves ; and in the very simplest Zoophytes (as the Hydra), we even find a capacity for reproducing the entire fabric to lie in every fragment of the body, just as a fragment of the leaf of Bryophyllum will give origin to an entire plant (§21, note). Mollusca It is and in many other particulars, closely borders upon Zoophytes. onlv among a very small number of the lowest Articulated animals, And among however, that this method of multiplication presents itself. Vertebrate it seems entirely wanting as a regular habit, although there is reason to think that it may occasionally occur as an abnormality, at that early period of the evolution of the germ when its grade of develop ment has not advanced beyond the Zoophytic stage (chap, xi.) 35 Underlying these well-marked types of Animal organisation, how- ever, there is a group of beings which cannot be regarded as presenting even a rudiment of the plan of conformation that is characteristic of any one of them, and in which scarcely any differentiation of organs is to be discerned,— a group, in fact, which holds a rank in the Animal kingdom, that is precisely parallel to that of the Protophyta in the Vegetable (§ 22), and which may therefore be appropriately designated Protozoa. Between these two groups, indeed, no definite line of demarcation can be drawn j and the same beings have been reckoned as Plants or as Animals, accord- ing to the particular views of the classifier in regard to the mode m which they should be distinguished. A large proportion of the Protozoa consist of single cells, or of aggregations of cells in which there is no differentiation of character j and in the lowest forms of them, there is not even that distinctness of the cell-wall from the cell-contents which exists in every completely-developed cell, but the whole forms one mass of living jelly (Fig. 33). The animal character of this, however, is marked in its' mode of nutrition ; for it does not draw its aliment, like the Proto- phytes, from the surrounding air and moisture, but is dependent for its support upon organic substances previously elaborated by other beings, which it envelopes with its own j elly-like substance, and of which it own * In the Author's opinion, the Zoophytes, not the Eehinodermata, are the types of the Radiated series ---Gasteropoda of the Molluscous;— Insects of the Articulated;— and Mammals of the Vertebrated. i h I 40 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. increase. The animal character of this body is also indicated by its movements : for although the ' zoospores' of the Protophyta and lower Fig. 33. /.■"•Oi'3; /-•,„'! CO or o t c 4V>Jo tY4>lf c ^H;^ C C 3 c ^ c r .u 4 4 c . t, r ° c **_'/% < •* v v cct. n y »i C v Ctf \ ^4 Amceba princeps, indifferent forms, a, b, c. AlgaB are rapidly propelled through the water by ciliary action, yet they do not exhibit that motion of one part upon another, which is often seen m the simplest Protozoa But there are as yet no special instruments either for sensation or for motion. As every part of the body is equally adapted for digestion, for absorption, for circulation, for respira- tion, and for secretion, so does every part appear equally capable of receiving impressions made upon it, and of responding to them by a con- tractile movement. From this starting-point we may proceed in either of two directions ; for we find in the Infusory Animalcules a tendency to the mdividualisation of the single cell, which seems to attain in them its highest development as a separate entity; whilst in the Rhizopoda ( Fora- mmifera) and Part/era (Sponges) we find aggregations of gelatinous bodies (which present more or less distinctly the characters of true cells) assum- ing certain definite types of form, and approaching the individuality of higher organisms-In the true Animalcules (excluding the Rhizopods and the Protophyta which have been confounded with them) we find an obvious distinction between cell-wall and cell-cavity ; there is a definite opening into the latter, through which food is introduced, instead of its being received into any part of the mass ; and there is frequently also a second orifice, through which indigestible particles are expelled ' More over the locomotion of these beings is performed, as in the Protophyta by the agency of aha; these being prolongations of the cell itself, to which the contractile power is especially delegated. Their multipl cation is ordinarily accomplished, hke that of the Protophyta, by duplicative sub- division; and L in this way a vast number of similar beings may be pro- duced, each of which is a repetition of the rest, and lives altogether inde- pendently of them. But it seems probable that, like the Protophyta, they have a proper generative process, consisting in the ' conjugation' of two similar cells ; no sexual distinction as yet manifesting itself between tnese, and both of them apparently contributing in the same manner and GENEKAL VIEW OF ANIMAL KINGDOM. PROTOZOA. 41 degree to the production of the germ. — In the Mizopoda, we find the simple j elly-like mass extending itself by gemmation, and at the same time very commonly forming a calcareous envelope upon its exterior; whilst through apertures in this are put forth extensions (pseudopodia) of the soft substance in its interior, through which the introduction of nutriment into the body seems to be chiefly effected. Notwithstanding the small amount of differentiation which appears to exist among the several products of gemmation, yet a strong tendency to individualisa- tion in the entire aggregate is shown in the very definite plan of growth which each snecies exhibits, as is most obviously seen in Nummulites Of the mode of multiplication and other higher forms of Foraminifera. of these animals, nothing is yet known. Porift there is, with less definiteness of configuration in the aggregate mass pro- duced by gemmation from the single primordial cell, a much higher degree of mutual interdependence; for we now find the component particles so arranged as to form the rudiments of differentiated organs, whilst the general plan of structure approaches that which we meet-with among the lower Zoophytes, in whose fabrics the individuality of the components is still more completely merged in that of the organism as a whole. For, in the first place, we have a marked distinction between the internal fibrous skeleton and the soft flesh which clothes it; and these components have a very definite and characteristic arrangement, which varies in different parts of the mass ; being dissimilar, near the external surface, and around the internal canals, to that which prevails in the intervening substance. Again, in the system of absorbent pores for the entrance of liquid, and of ramifying canals for its discharge, we have the first rudiment of a digestive and circulatory apparatus, not yet marked-off, however, from the general cavity of the body, the organs of nutrition do not present any further specialisation, yet those of reproduction are differentiated from them, and are limited to particular And although parts of the mass. Even in this lowest form of an aggregate Animal, true ovum is produced : so t] already advance to the same essential type of generation, as that which prevails in the highest plants. 36. Among the four definite types of structure under which all the higher forms of Animal organization may be ranked, the Radiated, as already remarked, unquestionably holds the lowest rank: in virtue alike of the close conformity of its general plan to that which prevails in the higher Plants ; of that predominance of its Vegetative or Nutritive apparatus over that of Animal life, which is conspicuous even m its higher types; and of that very imperfect differentiation of the organs of the former, which prevails through the larger part of the group. Each of these points will now be noticed in some detail. — The radial sym- metry must be regarded as in itself a vegetative character, for it cor- responds with that which is seen in the disposition of the appendages around the axis in the leaf-buds and flower-buds of plants ; and it is inti- mately connected with another vegetative character, the repetition of similar parts. Thus, in the animals in which it prevails, we find the central mouth to be surrounded externally by a circular series of pre- hensile appendages ; which may be mere oral tentacles, as in the Polypes (Figs. 34, 35), the Medusce (Fig. 93), and the Holothuria (Fig. 40), true R> • ; 42 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. arms Star-fish organs, a similar character is exhibited; that is, a circular disposition of parts which precisely repeat each other. There are, it is true, modifi- cations of the radial type in certain aberrant forms of the group, which tend towards a bi-lateral symmetry; but these are comparatively • rare exceptions, which it is only necessary here to mention. It is not only in their radial symmetry, however, that the animals of this division are conformable to the type of the higher portion of the Vegetable kingdom; for this conformity is equally shown by a large proportion of the group' structures From a . single polype, as from a single leaf-bud, an arborescent structure may be evolved, bearing hundreds or even thousands of polype-bodies, all origi- nating from the first, and maintaining an intimate organic connection with each other; thus bearing a close physiological resemblance to a tree, and requiring to be considered (like it) as a single individual, although its several members have no relation of mutual interdependence' and can maintain a separate existence if detached. It is not to be won' J dered at, then, that the older Naturalists, who were only acquainted with the skeletons of Zoophytes, should have considered them as vege- table structures, and that many of them should even now be popularly regarded in that light; whilst even the movements exhibited by the living polypes, not being apparently very different in nature from those per- formed by the Sensitive-Plant, or the Venus's Fly-trap, did not seem sufficient to establish their animal nature. This extension of the original fabric by gemmation may take place among Zoophytes to an indefinite extent ; and the mode in which it occurs is the chief determining cause growth is traceable in each species, but which is liable to great variation from the influence of external conditions. In nearly all the members of the class of A celephce, it seems to take place at some period of life or other ; for although we find few traces of it in the fully developed Medusae, yet (as will be shown hereafter, chap, xi.) multiplication by gemmation takes place to an extraordinary extent during the early stages of their existence; and in some of the lower forms of the group, especially those which closely approximate to the Zoophytic type, it continues during the whole of life, and gives rise to those composite fabrics of the Girrhigrade and Physograde orders, which, until the recent discovery of their true cha- racter, have been a source of so much perplexity to Naturalists. In the class E ' chinodermata, multiplication by gemmation very seldom takes place; but its members retain throughout their lives an extraordinary measure of that power of reproducing lost parts, of which the production of an entire organism by gemmation is only a higher manifestation. 37. The low development of the proper Animal powers in Radiated animals, as compared with their Vegetative activity, is one of the most remarkable features of the group taken as a whole; nor are there are any exceptions to this general character. In none of the true Zoophytes is the nervous system differentiated from that general fibro-gelatinous tissue of which the entire bodies are composed ; every part seems more or less impressionable and contractile, although these attributes are most strongly displayed in the oral tentacula; and there is no evidence that GENERAL VIEW OF ANIMAL KINGDOM. ZOOPHYTES. 43 » the respondence to external impressions which is probably the source of all their movements, proceeds from any distinct consciousness of these impressions. It is in the Acaleplm, that the first traces present them- selves of a nervous system, and of organs peculiarly fitted to receive sen- sory impressions; but it is probable that a large part of the movements executed by even these animals, are not dependent upon any influence transmitted through this apparatus. In the Uchinodermata, whose organs nervous is more clearly marked out; and the distinction between nerve-cords and ganglionic centres, which has not yet been clearly established in the Acalephse, may be unmistakeably affirmed to exist. There are also rudi- ments of eyes in certain members of this class; and there is some evidence that their movements are directed by visual impressions received through these organs. 38. Between the lowest and members of the Fia. 34. the highest Radiated series, there is a very marked contrast in regard to the differentiation of the principal organs of Vegetative life; but a number of intermediate grada- tions present themselves, which establish a tolerably complete transition from the one condi- tion to the other. — Commencing with the Hydra (Fig. 34), we find the digestive apparatus re- duced to a state of the greatest simplicity, the whole body seem- ing to be nothing else than a stomach, with a circle of pre- hensile tentacula around its ori- fice, which, being single, and serving alike for the reception of food and for the ejection of its indigestible portions, must be considered as representing in. itself the cardiac and pyloric orifices of the stomachs of higher animals. The wall ^ of this cavity and the general in- tegument of the body are so closely connected together, as to seem like two layers of one and the same membrane; there are, however, some lacunar spaces between them, constituting the first indication of that ' general cavity of the body which exists m r D a, Hydra fusca, or Brown Jfresn-water roiype, attached to a piece of stick, with its arms extended, as in search of prey ; a, the mouth surrounded by tentacula ; b, foot or base, with its suctorial disk : at b is seen a portion of one of the arms near its origin, and at c another portion near its termination, more highly magnified. in almost every other animal, and which performs, as we shall see, very important functions ; and these lacunar spaces communicate 44 DEVELOPMENT with similar cavities in the interior of the tentacula. There does not yet appear to be any decided structural or functional differentiation between the layer which lines the stomach and that which clothes the body; since each can perform all the offices of the other, as is shown by the result of Trembley's well-known experiment. No circulating apparatus is yet distinguishable, the nutritive liquid, which is the pro^ duct of the digestive operation, being at once absorbed from the parietes of the stomach into the general substance of the body and arms ; nor is there any special respiratory or secretory apparatus. Even the gene- rative organs, which are usually the first to be differentiated from the rest of the fabric, cannot here be distinguished ; for ovules and sperm-cells are evolved in the substance of the ordinary tissue; and the only indication of their specialization is afforded by the restriction of their production to particular situations, the sperm-cells usually making their appearance just beneath the arms, whilst the ovules protrude nearer the foot. The homogeneousness of the entire body, however, is most remarkably evinced m the facts, that gemmce which develope themselves into new Hydrse sprout almost indifferently from any part of it, and that a minute frag- Fig. 35. ment from any region will (under favourable circum- stances) regenerate the whole. In the composite fabrics which are formed after the Hydra- form type (Fig. 99), the conso- lidation of the external in- tegument necessitates several other changes ; amongst the rest, the evolution of a special reproductive apparatus, and the separation (within the polype-cells) of the wall of the stomach from the external integument, so as to com- mence the formation of the 6 general cavity of the body. 5 This, however, is carried much further in the Actinia (Fig. 35), and in all the Polypes formed upon its type ; for in these we find the stomach suspended (as it were) in a large space, which is subdi- ,.,...,_! , , vided by radiating partitions; and it is m the chambers thus formed (which are prolonged into the interior of the tentacula) that the generative apparatus is situated. Yery distinct organs for the production of sperm-cells or of ova are here evolved : these organs (according to late researches, chap, xi.,) not being combined in the same individuals. There is still a direct connection between the interior of the digestive sac and the general cavity of the body, by an aperture at tJie bottom of the former ; and through this, the nutritive products of digestion find their way into the surrounding cavity, mingled with the Diagrammatic section of Actinia, showing its interna structure ;— a, a, base or foot ; b, b, oral disk ; c, c, tenta- cula ; d, mouth ; e, stomach ; g, 9i k, Jc, vertical partitions cut across m different directions ; g', g', apertures in these: ft, passages opening into the tentacula ; L L testes or ovana; m } m 3 filiferous filaments. f- GENERAL VIEW OF ANIMAL KINGDOM. — ACALEPH^. 45 water which is introduced through the mouth. This is the only mode in which the tissues are nourished, as there is not yet any special circulating apparatus ; and, in like manner, it is only by the expulsion of the fluid that has remained for some time in the general cavity, that the excre- tory products which have found their way into it from the tissues, can be carried out of the body, in those species which have no orifices at the extremities of the tentacula. Thus the very same liquid answers all the purposes, in these simply-formed animals, which are served in Yertebrata by chyme, chyle, arterial blood, and venous blood; and it also serves as the external i a medium for respiration, tegu thickened and hardened, that the amount of aeration of the interstitial fluids which takes place through it must be extremely limited, in com- parison with that which will be carried on through the delicate mem- branes clothing the internal surfaces. Thus, with some very important points of differentiation, the general type of these animals remains extremely low ; and their power of multiplying by gemmation, and of reproducing lost parts, in which they are only inferior to the Hydra, is what we might anticipate from their general homogeneousness. In the Actiniform Zoophyt remains between the general cavities of the Polypes which have budded-off one from another ; but this connection is more intimate in the A Icyonian Zoophytes (Fig. 100), among which the 'general cavity extends through- out the polypidom, forming a branching system of canals which strongly resembles that of Sponges. In fact, when we compare the two organisms, we can scarcely fail to perceive that the Alcyonium is essentially a Sponge of which certain parts have been differentiated from the rest, and evolved into special organs. And this view is confirmed by the circumstance, that when a bud is put forth from one of those polypidoms, it has all the ordi- nary characters of a Sponge, except that its canals do not open upon the external surface (Fig. 91) ; the formation of a polype-mouth and stomach not taking place until a later period. 39. The lower forms of the class of Acalephce carry us back to the grade of development proper to the composite Hydraform Zoophytes. But in the higher, such as the ordinary Medusa (Fig. 36), there is a far less amount of repetition of similar parts, the gemmae detaching themselves from each other at an early stage of development, and subsequently maintaining an entirely independent existence. There cannot be here Hvd said, any more than m the v m _ space between the walls of the digestive sac and of the ovarial chambers which surround it, and the external integument, is occupied by homo- geneous solid tissue. But a series of gastro-vascular canals, commencing from the stomach, radiates towards the margin of the disc ; and these serve the double purpose of conveying the nutritive product of the diges- tive operation to the remoter parts of the body for the supply of their wants and of subjecting it to the aerating influence of the surrounding medium. In its return to the centre, the fluid will of course carry back with it whatever excretory products it may have received from the tissues through which it has passed ; and thus, like fluid of the stomach and general cavity of the Actinia, it answers to the chyme, chyle, arterial blood, and venous blood, of Yertebrated animals. In the Beroe (Fig. 102), and certain allied forms, the digestive cavity has an anal as well as an oral : ■ * 46 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. orifice; and there also appears reason to think, that in its system of gastro- vascular canals a difference already exists between the afferent and efferent tubes, the fluid passing forth from the stomach by one set, and returning to it by the other. The generative apparatus in this class always exhibits Fig. 36. Structure of Cyanosa aurita. — Disk seen from above, showing the quadrilateral mouth a, the four ovaries bbbb, the four orifices of the ovarian chambers cccc, the stomach dddd, and its radiating prolongations, the eight anal [?] orifices e e, &c, and the eight ocelli [?]//, &c. * a very well-marked differentiation; its type being in many respects Medusa round own species of A ctinia the extremities of whose tentacula are closed) the only channel for the escape of the fertilized ova or of the rudimentary youi The sexes are here distinct, the ova and testes not being combined in the same bodies : and this is true also of many of the composite forms which develope medusa-like buds containing sexual organs, each indi- vidual producing buds of only one sex, as in dicecious plants : in others however, male and female medusa-buds are developed on the same stock as in monoecious plants, although in no case are the two sets of genera- tive organs combined in the same medusoid body. 40. In the class Echinodermata, the Asterias (Fig. 37) holds by no means an elevated rank; yet we find in it a very marked advance upon either of the types previously described. The stomach with its single orifice, suspended in the midst of the < general cavity of the body,' reminds us of that of Actinia ; but it is entirely cut off from that cavity, which consequently remains closed. The nutritive products of digestion probably find their way into it, however, by transudation through the m^m --* n * « GENERAL VIEW OF ANIMAL KINGDOM. — ECHINODERMATA. P +,Tip «+,nTrm.p.li • and it is the/n^e taken ui) bv a regular 47 vessels, the distribution of which, however, is very limited, so that the Fig. 37 "vr Asterias awrantiaca, with the upper side of the hard envelope removed :— a, central stomach; b C£eca upon its upper surface (sah vary glands ?) ; cc, csecal prolongations of the stomach into rays -c' the same empty ; d, the same laid open; e, the under surface, seen from within after the removal of the cseca, showing the vesicles of the tubular cirrhi ; /, the same in a contracted state, showing the skeleton between them. fluid of the general cavity seems still to take the largest share in the nutritive operation. It is interesting to remark, that in this class we already meet with a differentiation, however imperfect not only between the fluid of the gastric cavity, or chyme, and that of the surrounding visceral cavity, or chylaqueous fluid * but also between the latter and the * The term chylaqueous fluid, introduced by Dr. T. Williams, appears to the Author to be well adapted to designate the fluid of the 'general cavity,' when (as in Echinodermata and Annelida) this is distinct alike from that of the digestive sac, and from that of the proper circulating system. It is far more extensively employed, however, by Dr. Williams m his ingenious Memoir « On the Blood proper and Chylaqueous Fluid of Invertebrated Animals' in the " Philos. Transactions/' 1852; being there applied ^ the immediate product of gastric digestion which passes directly into the ' general cavity' of the Actini- form and Alcyonian Zoophytes, and even to that which is confined within the stomach and gastro -vascular canals of Medusae. ! 48 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. blood contained within the proper circulating system. A special pro- vision appears to be made for respiration in these animals, by the trans- mission of the ' fluid of the general cavity' into a multitude of short delicate csecal tubes, which pass between the pieces of the calcareous Fig. 38. Comatula rosacea. cilia that keep up a constant movement in their contents. Fig. 39. The genera- JEchinus mammillatus. are lined with And there are various secretory or- gans possessing a dis- tinct glandular charac- ter, whose special uses are not yet certainly known. tive apparatus here attains a high develop- ment, the ovaries and testes (as in the higher Acalephae) being no longer combined in the same individuals, and having separate orifices discharge of for the their products ; it is in- teresting to remark, however, that in Comatula (Fig. 38), whose digestive apparatus is framed upon a higher type than that of Asterias, the ovaries are dispersed in isolated spots through the integument of the arms. The Star-fish exhibits a series of elaborate provisions for locomotion, in the beautiful articulation of the plates of the calcareous skeleton, in the con- tractility of the general integument of the body, by which its lobes (misnamed < arms') are moved in various directions, and in the multipli- h fc «n* GENERAL VIEW OF ANIMAL KINGDOM. ECHNINODERMATA. 49 cation of tubular cirrhi furnished with suckers, by the contraction of which, when the suckers (forced out by the injec- tion of fluid into the cirrhi from the e general ca- vity') have taken an attachment, the body is drawn towards the points to which they have adhered. — The chief feature of advance in the Echinus (Fig. 39) is the conversion of the digestive sac with a single orifice, into an alimentary canal with a separate mouth and anus; and around the mouth we find a very elaborate dental apparatus, furnished with distinct muscles, such as do not make their ap- pearance m any lower forms of or- ganisation. The locomotive appa- ratus, too, is still more highly de- veloped ; for the body being now enclosed in an im- movable case, so that its parts are not themselves capable of flex- ure, a new set of instruments is evolved, namely, the calcareous Fig. 40. a Anatomy of Holofhuria tubulosa ; — a, anus ; b, mouth, surrounded by 20 tentacula ; c, cloaca, surrounded by muscular dilators c'; i, intestinal tube ; m f mesentery ; ml, ml> longitudinal muscles ; mt, transverse muscles lining the entire inner surface of the integument ; o, ovary ; ap, csecal appen- dages, probably seminiferous ; p, contractile vesicle, probably a heart ; r, r, respiratory apparatus, originating in the cloaca; t, oral tentacula ; t', csecal reservoirs; va 9 annular vessel surrounding the mouth and supplying the ten- tacula ; ve, external intestinal vessel, giving off a large anastomotic branch va' which enters another part of the same trunk ; vi, internal intestinal vessel, with contractile dilatations ; vl, longitudinal tegumentary vessel, giving off transverse branches vV, seen by removing the longitudinal muscles ; vm, mesenteric vessels, connecting the branches of the external intestinal vessel with those of the respiratory system of vessels, vr. E ■ m 50 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. | I spines, which project from the surface, and are put in motion by the contractile integument, upon the ball-and-socket joints at their base. — The Holothuria presents us with certain interesting features of more complete differentiation, without, however, any very decided advance upon the type of the Echinus. The absence of a solid 'test' enables its movements to be performed by the flexure of the body generally; and for this a regular series of longitudinal and transverse muscular bands (Fig. 40, m I, m t,) is provided, reminding us of those of the Worm-tribe. The alimentary canal (i) does not yet present any distinc- tion of parts into oesophagus, stomach, or intestine, but remains of nearly the same diameter throughout its length; it is held in its place in the midst of the general cavity of the body, however, by a regular mesentery, upon which the blood-vessels are minutely distributed. The circulating system is more complete than among other members of this class, especially in its peripheral portion; and it is furnished with a pulsatile vesicle Qp), whose contractions assist the onward movements of their fluid. For respiration there are two special provisions ; the fluid of the circulatory vessels being aerated by transmission to the branching oral tentacula (t) ; whilst that of the * general cavity' receives the same in- fluence from the water introduced through the respiratory tree (r, r). The restriction of the outlet of the genital apparatus (o) to a single aperture (the five equal and separate portions of this apparatus in the Echinus and Asterias having each its own outlet) is a very decided character of elevation ; which seems to have been presented also by the extinct group of Cystidea (Fig. 81), notwithstanding that in the attachment of these animals by a stalk to a fixed basis, they (in common with the Crinoidea) showed a decidedly zoophytic tendency. 41. The Molluscous sub-kingdom, like the Radiated, is remarkable for the high development of its apparatus of vegetative life in comparison with that of animal life ; but its type of conformation is altogether dif- ferent. It is true that, in the lowest group of this series, there is such a close apparent conformity to the Zoophytic type, that the animals belong- ing to it were, until recently, unhesitatingly ranked under that designa- tion. But it is now perceived that the resemblance is only superficial; being dependent, in part upon the mode in which these animals extend themselves by gemmation, so as to form arborescent structures very ana- logous to those of true Zoophytes ; and being partly caused by the state of degradation to which various organs are reduced, whereby their true type is obscured. — Taking it as a whole, the Molluscous series is charac- terized rather by the absence, than by the presence, of any definite or symmetrical form. In the Zoophytoid Mollusks, it is true, we are reminded of the radiated type by the circular arrangement of organs around the mouth (Fig. 49, a); whilst in the family of Chitonidce, we meet with a division of the external skeleton into segments (Fig. 41), which reminds us of the articulated type. But these are peculiar excep- tions ; and a Molluscous animal is essentially a bag of viscera, enveloped in a skin which is thickened in parts by muscular fibres that are not arranged after any constant plan. In the 'archetype' Mollusk, the mouth and anus are situated at the two extremities of the sac ; and the various organs are disposed symmetrically on the two sides of a longitudinal i i ■»■■ 'i ! GENERAL VIEW OF ANIMAL KINGDOM. — MOLLUSC A. 51 Fig. 41. A B median plane, just as in a Vertebrate or Articulate embryo ; the centres or principal trunks of the I circulating apparatus being on the dorsal aspect (which may hence be termed the ' haemal'), whilst the prin- cipal centres and trunks of the ner- vous system are on the ventral aspect (which may hence be termed c neural'.) But this simple and symmetrical arrangement is very commonly obs- cured by subsequent inequalities in the development of particular regions, so that an entire change takes place in the relative position of the dif- ferent organs, and the types of con- formation thus evolved seem to have little or no affinity to one another. * — The nearest approach to the archetype is presented on the whole by those of the Tunicata, in which the two orifices retain their original positions at the poles of the body (Fig. 42) ; and their chief peculiarities consist, in the A, Chitonellus. — B, Chiton. arynx Fig. 42. Salpa maxima; a, oral orifice; 5, vent; c, nucleus, composed of the stomach, k™ r > & £j * branchial lamina ; e, the heart, from which proceeds the longitudinal trunk/, se ?J™e ™£?" verse branches across the body ; g K g, projecting parts of the external tunic, serving to unite the different individuals into a chain. branchial sac, and secondly, in the inversion of their integument around the anal orifice, so as to form an immense cloacal cavity, the wall of which extends so far into the interior, and so completely envelopes the general mass of the body, as to constitute what is known as their c inner tunic. 't Mollusks • J W [ij [VyJ llXIlVi- «J U*-JLJL-I-^/» F ■■ ■ ■ ■ '■ *— — - y • principal extension of the integument takes place externally ; a duplica- L™ n f +liP tlii ok en ed glandular skin of the < dorsal' or ' haemal' region ture of the thickened glandular skin of the ■ * See Mr. Huxley's admirable Memoir < On the Morphology of the Cephalous Mollusca,' in the " Philos. Transact. 1852." , t Such is Mr. Huxley' s very ingenious account of the production 01 tins tunic, as given in his "Report on the Tunicata" to the British Association, 1852. See also his Memoirs 4 On the Anatomy of Salpa and Pyrosoma,' and his ' Doliolum ' in the " Philos. Transact." for 1851. - ' E 2 Remarks upon Appendicularia and ^n^^^m-^^m ! ! r i 52 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. (here termed the mantle) being prolonged on either side into two lobes, which enwrap the body like a cloak, and form the valves of the shell upon their outer surface (Fig. 43). Again, a special development of muscular Fig. 43. Dorsal Margin. • rH a X o 3 h3 o 02 S. 8 ^ *- ■'"'""■■( r-trrrrm mtmamttf"/' mu ^ Ventral Margin. Anatomy of Mactra ; — a, anus ; b, posterior muscle ; c, branchial ganglion ; d, ovary ; e, t, intestine ; f, shell ; g, nervous cord, connecting oesophageal and branchial ganglia ; h, stomach • i, heart ; Jc, liver ; I, anterior or oesophageal ganglia ; m, anterior adductor muscle ; n t nervous filaments ; o, mouth ; p, one of the oral tentacula ; q x 3 mantle -, r, margin of the shell ; s foot : w, branchial lamellae ; y, oral siphon ; z } anal siphon. inteomment ventral' or c neural' region constitutes the ' foot' of those Lamellibranchiata which possess such an organ. — In the Gasteropoda this foot assumes the form of an expanded disk (Fig. 44, a\ upon which the animal can crawl ; the two extensions of the upper part of the integument are wanting ; but the form of the body itself is entirely altered by the extraordinary and commonly unsymme- trical development of the hindmost portion of the haemal region into a 6 post-abdomen/ which contains the heart and a considerable part of the alimentary canal, and from the mantle of which a shell (Fig. 45) is very frequently produced. In the pulmonated Gasteropods (Fig. 124), however, the development takes place before instead of behind the anus ; so that an ' abdomen' is formed instead of a post-abdomen. — This is also the case in the Pteropoda, in which the < foot' proper is but little developed, whilst two lateral expansions (epipodia) sent off from it constitute the wing-like appendages (Fig. 46) from which the group receives its designation. Finally in the Cephalopoda, the abdomen is so peculiarly developed that the alimentary canal is quite doubled upon itself, so as to bring the anus into immediate proximity with the mouth (as happens also in the Bryozoa at the opposite extremity of the series) ; the margins of the foot GENERAL VIEW OP ANIMAL KINGDOM. — MOLLUSC A. 53 are prolonged into those prehensile processes (Fig. 47) which are termed < arms ;' and the posterior epipodial lobes, by their cohesion, form the Fig. 44. J t t f i * Paludina vivivara withdrawn from its shell and laid open ;— a, foot ; b, operculum ; c, one of thf ScuCfth tts Melius ; d, siphon ;/, border of the mantle ; .^pectmated branchy ; *£ tbe oviduct dilated for the retention of the ova ; k", portion of it situated within the spire 01 tne sheUi termination of the intestine; I, canalfor the urinary (?) secretion; » heart ;o, liver Tvroho^Tq' ° oesophagus ; r, stomach ; f , «' s", intestine, lying at * within the branchial cavity Tu, u, clpnalic ganglia ; v, v, salivary glands ; x, the principal muscular nerve. funnel' that serves for the discharge of the respiratory current and of the matters ej ected from the intestine.— Thus each of the subordinate types that we recognize in the Molluscous series, presents us with its own Fig. 45. A B C I> Shells of Gasteropod MollmU .— Aj Achatina;-B, Sigar etus ;-c . Vermetus ;-n, Scalaria. j ■ 54 GENEKAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. special character of differentiation from the general ' Archetype :' and there is no real transition from the one to the other.* Fig. 46 ; Fig. 47 Existing forms of Pteropods.— a, Hyatea; b, Criseis j c, Clio* Sepia officinalis , or Cuttle-fish. Fig. 48. 42, Turning now to the internal organization of the animals of the Molluscous sub-kingdom, we find that the alimentary canal almost in- variably presents a distinct separation between the oesophagus^ the stomach, and the intestinal tube ; this separation being as obvious in the zoophytoid Laguncula (Fig. 49), as in the Gasteropod Aplysia (Fig. 50). The mouth, or entrance to the oesophagus, is not situated, in the lower Mollusca, on a prominent part of the body, nor is it surrounded by organs of special When it was first discovered that the embryo-forms of Gasteropods (Fig. 48) possess a pair of ciliated lobes corre- sponding in general position with those of Pteropods, the notion was entertained by many, that the animals of the latter group must be considered in the light of permanent embryoes of the former : this, however, is incon- sistent with the fact pointed out by Mr. Huxley, that the ciliated lobes of the embryo Gasteropods are homologous with the anterior portion of the epipodium, whilst it is the middle portion alone which is developed into the ' alse' of Pteropods ; and that a more fundamental distinction lies in the development of an ' abdo- men' in Pteropods, whilst it is a 'post-abdomen' which is deve- loped in Grasteropods. Embryoes of Nudihranehiate Gasteropoda GENERAL VIEW OF ANIMAL KINGDOM. MOLLUSC A. 55 sense ; and hence these are distinguished classes, however, it is situ- ated on a head, which pro- jects from the trunk, and which is usually furnished with well-developed eyes, and with rudimentary or- gans of smell and hearing. In the lower Mollusca, want of acephalous. In the higher Fig. 49. ^fe again, there is a any prehensile or reducing apparatus, the food-particles being drawn-in by ciliary currents, which are also subservient to the respira- tory function; but in the higher, the mouth is fur- nished with a complex ap- paratus for the reduction of solid food (Fig. 50, a), and prehensile instruments are added in the Pteropods and Cephalopods. In addi- ,, a portion of the stomach is frequently developed into a gizzard- structure, with firm walls, adapted tion to thi s like very still further to crush and com- minute the food ; and this is found in many Bryozoa, as well as in several Gas- teropods (Fig. 50, i) and in Cephalopods generally. The liver is always reco- gnizably present; and al- though in the Bryozoa it consists of nothing else nothing than an assemblage of iso- lodged lated follicles, lodged in the walls of the stomach (Fig. 49, b, A), yet as we ascend the series, we find it gradually becoming more detached from and in the and more that organ ; higher Mollusks it is de- veloped into a compact viscus (Fig. 50, Z, I), which Laquncula repens, as seen in its expanded state at a, and in its contracted state, in two different aspects at b and c-llie same references answer for each figure :-a a, ^^^^ with vibratile cilia; 6, pharyngeal cavity; e, valve gating this cavity from d the oesophagus ; e, the J^tthTov£T' r>vloric valve and a the circle of cilia surrounding tnat ormce, V waU oi 'the stomach with biliary follicles; *, the intestine, containing excrementitious matter, and terminating at I, ?hf anus m, the testicle ; n, the ovary ; o, an ovum set free frnrHhe ovarv; P, openings for the esape of the ova; q, spermatozoa freel/moving in the cavity that surrounds the viscera • r, retractor muscle of the angle of the aperture of the sheath :' *, retractor of the sheath ; t, retractor of the tenta- cular circle ; u, retractor of the oesophagus ; v, retractor of the stomach; w, principal extensor muscle; x, transverse wrinkles of the sheath ; y, fibres of the sheath, themselves pro- bably muscular ; z, muscles of the tentacula ; a (at the base of the tentacular circle in a) nervous or oesophageal ganglion ; q s tem.— D, a portion of the tentacular circle shown separately on a larger scale ; a a, the tentacula clothed with cilia : b b, their internal canals ; c, muscles of the tentacula ; d, trans- verse muscles forming a ring at the base of the tentacula ; e, muscles of the tentacular circle. frequently bears a very large proportion to the general mass ot the body. As we ascend from the lower to the higher parts of the series, moreover, 56 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. we find other secreting structures connected with the alimentary canal, such as the salivary glands and pancreas, presenting themselves in a more Fig. 50. // / it i /■ I !7 - - i V r s / Aplysia laid open, to show the arrangement of the viscera :— a, upper part of the oesophagus : b, penis j c, c. salivary glands ; d superior or cephalic ganglion; e, e, inferior or subcesophageal ojangha ^entrance of the oesophagus into g i9i the first stomach or crop ; h, the third or true iigestiye stomach ; i, the second stomach or gizzard; Jc t intestine ; I, I, I, liver ; m, posterior or branchial ganglion; rc aorta; o hepatic artery; p, ventricle of heart; a, auricle: r, s, branchiae : t 3 testis ; u, lower part ot intestine ; v, ovary ; w, anus. and ^ more specialized condition ; and the general type which these attain in Cephalopods, closely approximates to that under which we find them in Fishes. In no instance does the alimentary canal possess any direct I t GENERAL VIEW OF ANIMAL KINGDOM. — MOLLUSCA. 57 communication with the ' general cavity ot tne ooay, in xne mm*.* ^ which it is suspended ; but it may be affirmed with certainty that a transudation of nutritive material takes place through the walls ot tne former into the latter, and that this is the channel through which this material finds its way into the circulating system. For in the Bryozoa, there is absolutely no other means by which the body at large can be nourished, no true circulating apparatus existing in this group ; so that the extension of the visceral cavity throughout the body, and even into the tentacula and stalk, constitutes the sole means by which the products of the digestive operation can be applied to the nutrition of the parts remote from the alimentary canal. Where a distinct vascular system exists, it communicates freely with this « general cayity of the body; so that the blood in one part of its circulation is freely discharged into it. In the higher Mollusks, however, the viscera themselves occupy so large a proportion of this cavity, that the remaining space is greatly re- duced in size, and presents so much of the character of an ordinary venous sinus, that its true nature has not been until recently discovered. Not- withstanding this very important feature of degradation, we find the heart or central impelling organ of the circulation rapidly becoming more and more specialized as we ascend the series. No trace of it, of course, is to be found in the Bryozoa ; in the Tunicala it is generally but little more than a pulsatile dilatation of one of the principal trunks (Fig. 42, e), Conchife with tinction becomes still more strongly marked, both structurally and physio- logically, in the higher classes. With ywher aeration of the circulating fluid by means of a distinct respiratory apparatus : but the position of this varies more than that of any other organ in the body ; and it is seldom that any other means are provided with the respiratory movement of the cilia with which it is clothed. urinary Mollusca the comes distinctly recognizable in the higher. 43. Not only the Bryozoa, but by far the larger proportion ot proper Tunicata, possess a capability of multiplying by gemmation ; decree of connexion, however, that continues to exist between the gemmae and the stock from which they have been put forth, varies m different groups. Thus in the Bryozoa a continuity is gently pi^serve d ; as in Laguncula (Fig. 49), between the 'general cavity gj^™* °\ °» e ZSoid* and another, through the whole of life; m Perop hora (Fig. 138), the continuity is maintained by the vascular system, which is here m such free communication with the general cavity of the body that it may be almost regarded as a prolongation of it; m BotryUus (*ig. 01), the buds are formed in the first instance by an extension of the general cavity of the body ' of the stock, but when they have attained an ad- vanced stage of development they become entirely separated from it and from each other, although still enclosed within a common envelope ; * Th e term Zooid has been suggested by Mr. Huxley, as an appropriate designation for each of the independent and self-maintaining organisms, which collectively result from a single generative act. 58 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. I I I I and in Salpa (Fig. 42), the buds developed from the ' stolon ' or creeping Fig. 51. # Botryllus violaceus : — a, cluster on the surface of aFucus: tion of the same enlarged. b, por- i stem in the interior of the stock, become detached at a very early period, and swim forth freely, although connected into chains by the mutual adhe- sion of their bodies. No multiplication by gemmation is known to exist in any of the higher Mollusca; but a peculiar generative zooid is detached from the male of certain Cephalopods, to con- vey to the female his spermatic fluid. — The true generative appa- ratus is very distinctly evolved throughout the series. In its lowest classes, the two sets of organs are united in the same individual, in such a manner that its 'sperm-cells' may impregnate its ' germ-cells,' and thus produce fer- tile ova, without any special operation. In certain of the Bivalves, however the sexes are distinct; but the fertilization of the ova is provided-for without any special congress of two individuals. In the Pulmonated Gasteropods both sets of organs are present in each individual, but they are not usually self-impregnating ; for the generative act is ordinarily effected by the con- gress of two individuals, each fertilizing the ova of the other by means of a highly-developed intromittent apparatus. In all the other Cephalous Mol- lusca, the sexes are distinct; and a regular sexual congress usually takes place. 44. ~No part of the organization of Molluscous animals exhibits the principle of differentiation more remarkably, than does the Nervous system. For whilst, in the Bryozoa and Tunicata, we find it to possess but a single ganglionic centre,^ which answers all the purposes required by the low de- velopment of their animal functions, a progressive multiplication of gan- glionic centres manifests itself as we ascend the series ; this multiplication not being dependent, as in Kadiata and Articulata, upon the repetition of similar ganglia (save in certain special cases), but having reference to the greater variety of purposes which this system is called-on to effect, chiefly in virtue of the development of more special organs of sensation and motion. In the Acephalous Mollusks, there is an almost entire absence of visual organs, and no trace of auditory or olfactive ; and the movements of such of them as are not fixed to one spot, are of the simplest and least varied nature, being effected either by the agency of the ciliary currents, or by general contractions of the muscular sac, or (in the Bivalves) by the con- traction of those special collections of muscular fibres, which constitute the adductor muscles and foot. In the Cephalous Mollusks, the rudimentary eyes found in some Acephala are progressively developed into organs fitted for distinct vision ; rudimentary organs of hearing begin to show themselves, which are evolved among Cephalopods into a proper auditory GENERAL VIEW OF ANIMAL KINGDOM. ARTICULATA. 59 apparatus; indications of a specialization of a part of the surface for olfactive purposes are also perceptible ; and conj ointly with this advance in the sensorial apparatus, we find the capacity for locomotion,— which is so feeble in most of the Gasteropods, that the term sluggish' derived from one of the best known members of that class is applicable to the whole of it, greatly augmented in the Pteropoda and Cephalopoda, many of the latter being nearly as active as Fish. 45. The plan of construction presented to us in the assemblage of animals constituting the sub-kingdom Articulata, is much more definite than that which we have traced through the Molluscous series ; and its leading features are in general more easily recognized, since the depar- tures from the i archetype ' form are seldom such as to interfere with the manifestation of its fundamental idea. Thus even in the Oirrhipeds (Figs. 4 5) which constitute its most aberrant group, the Molluscoid charac- ters are superficial only, whilst the prevalence of the Articulated type through every part of the internal organization is at once revealed by anatomical research.— The body of every Articulated animal is composed of a succession of segments arranged longitudinally ; the division being usually indicated externally by a differentiation in the consistence of the tegumentary skeleton, and by the repetition of the appendages (where such exist) which each segment bears. There is a manifest predominance, in the greater part of the series, of the organs of animal life over those of organic or vegetative life ; for the apparatus which is subservient to the locomotive powers, occupies, in all the higher Articulata, a very prominent position ; and it is kept in a state of high activity under the guidance of senses of remarkable acuteness. As it is by the external skeleton alone, that fixed points can be afforded for the attachment of the muscles and for the fulcra of the levers by which motion is given to the body, the degree of its consolidation generally corresponds with the development of the locomotive apparatus ; the chief exceptions being presented by those cases in which (as in the common Crab) there is an extraordinary development of a solid test for the purpose of protection only.— In some of the lowest grades of this type (belonging to the group of Entozoa), the successive segments which are indicated externally by constrictions of the body, so exactly repeat each other, that each can maintain an independent exist- ence, and can reproduce the entire body by gemmation ; so that, being indefinite in number, and physiologically distinct, they are nearly on the same footing with the independent zooids of a Botryllus. It is interest- ing to remark, moreover, that among these the Molluscous nature so far predominates/that there is scarcely a trace of locomotive organs, and the integument is soft throughout, so that the segmental division is chiefly indicated by the repetition of the organs of nutrition and reproduction. Ascending to the Nematoid Worms (Fig. 52), we find the segments united into one continuous body, not only externally but internally; and in these we find some of the leading features of the Articulated type dis- played in their simplest condition. The body does not externally pre- sent any true annulations, but a transverse wrinkling of the integument is generally perceptible ; and the muscular fibres which line this integu- ment are disposed in regular transverse bands, which are often sufficiently developed to endow these animals with a power of active movement. The oral orifice (a), situated at one extremity of the body, leads to an I : ■ 60 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. Fig. 52. alimentary canal {c y c,c), which runs through the axis of the cylinder to I the anal aperture (d) at its op- posite extremity ; presenting in its course scarcely any differen- tiation of parts into oesophagus, stomach, or intestine, and no distinct glandular appendages. There are two pairs of longitu- dinal vessels, one of which is supposed to be arterial and the other venous ; but there are not yet any special respiratory or- gans, the low degree of aeration which their circulating fluid requires, being still attained through the intermediation of the soft integument. The ner- vous system chiefly consists of a pair of longitudinal trunks which run along the ventral region of the body, and are connected with a pair of very minute gan- glia on each side of the oesopha- gus; but no distinct ganglionic enlargements present themselves in the course of these trunks; nor is there the least indication of the presence of organs of special sense. The two sexual divisions of the generative ap- paratus, which are not only combined in the lower Entozoa in the same individuals, but are even repeated through their suc- cessive segments, are here as- signed to distinct individuals; and the form alike of the ovary (e, e, e) and testes usually participates in the general elongation, which may be considered as resulting (like that of the digestive and vascular apparatus) from the ' fusion ' of the parts proper to each segment. — Hence the differentiation of parts is here almost at its minimum ; for there is nothing that can be properly termed a head ; and with the exception of the two terminal segments and that which contains the genital orifice, there is scarcely one that differs from its fellows in any essential particular of external configuration or internal structure. 46. In the class of Annelida we meet with a decided advance in the degree of specialization of the several organs, both of Animal and of Vege- tative life, without, as yet, any marked differentiation of the regions of the body, which is usually composed of a large number of segments pre- senting a close external resemblance (Fig. 53, a). In the lower forms, which nearly approximate to the higher Entozoa, the segmental division is obscured by the general softness of the integument, and by the absence Strongylus gigas (female) laid open to show its inter- nal structure ; — a, mouth ; b, oesophagus ; e, c, c, intes- tinal canal ; d, anus ; e, e, e s ovary ; f, uterine dilata- tion ; g, narrow oviduct ; h, its orifice. 1 ■ GENERAL VIEW OF ANIMAL KINGDOM. ANNELIDA. 61 rings with soft Fig. 53. of locomotive or branchial appendages ; but in proportion as these are developed and as the integument becomes consolidated, the annulose character is made obvious, by the alternation of firm intervening membrane. So, again, in the lower forms of this group, the head is scarcely more differentiated from the body than it is in Nematoid Entozoa; whilst in higher, it is furnished with proper eyes and antennae, and the mouth is B the the D an- genera ; H >^ s. 3** ?* I * J W \> HfcVjji, ft httli ti'itr. aasw m 71 )K y>. tw ' F F*"f 1 <'■;#('" \m , J k:- ■ §: •f-lIJht '(''■lift n * : & :'■ V / m % h5a Hi \ii i-£ *y h " *i / icU x ___ with consisting either of one, two, or three pairs of jaws, or of an evertible proboscis, for the prehension and re- duction of food (Fig. 53, b, c). Where locomotive appendages are developed, as in the tribe of Nereids, they are almost precisely repeated from one end of the body to the other; and this is the case, also, with the respi- ratory organs, save where the condi- tions of existence require that these should be especially developed from some particular region, as is the case with the cephalic branchial tufts of the Sabella (Fig. 144); in fact, the re- spiratory and locomotive organs in this group are by no means com- pletely differentiated from one other, each being subservient in great- er or less degree to both purposes. These appendages vary in different but we usually find them based on two fleshy tubercles on either side of each segment, which are termed the ' dorsal oars ' (Fig. 5 3, d, a'), or the < ventral oars ' (b'), ac- cording as they project from the upper or under half of the segment. Each oar commonly possesses, attach- ed to the base of its tubercle, a long soft cylindrical appendage, or cirrhus (d, a, e), homologous with the anten- niform appendages of the cephalic segment; whilst its summit bears a tuft of setce or bristles, which serve as the instruments of locomotion when the animal is crawling over solid surfaces. In the ordinary Nereids, more- over, each oar has also a membranous lobe (d, hj\ which is its instrument of propulsion in water. All these appendages, together with the pre- '/& & wwm i :-< !*■• S5 ■« *i; ^1 *»1 -m*\ \ 1 V"] X A, Nephthys Ilomlergii; B, its proboscis; c, the same laid open, to show the horny teeth it contains ; D, one of the feet, showing a', dorsal oar ; b', ventral oar ; a, dorsal cirrhus ; b, mem- branous lobe of dorsal oar ; c, tentaculiform ap- pendage; d, branchial appendage; e, ventral cirrhus ; f, membranous lobe of ventral oar. 62 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. ■ hensile and tactile organs which are developed around the mouth of many species, are probably subservient in greater or less degree to the aeration of the nutritive fluid transmitted to them ; but a more special respiratory organ (d), consisting either of a flattened vesicle or of a branching tuft, is developed from the under side of the dorsal oar ; this is not usually repeated, however, through the entire length of the body. 47. Notwithstanding the elaborateness of the buccal apparatus, the alimentary canal still retains much of its primitive simplicity, being an almost straight tube without any obvious division into oesophagus, stomach, and intestine; it is, however, furnished with csecal appendages of various kinds, apparently of a glandular character. The condition of the sanguiferous system in this group is very peculiar; for whilst in the higher Articulata, as in the Mollusca, it communicates freely with the general cavity of the body, so that one and the same fluid circulates through both, the blood-vessels here form a completely closed circuity as in the Echinodermata; and the general cavity is occupied by a true i chylaqueous ' fluid, which is kept in pretty constant motion by the movements of the body. As in the Echinodermata, too, there appear to be distinct provisions for the aeration of the blood-proper and for that of the chylaqueous fluid ; for whilst the latter penetrates into the locomotive, prehensile, and tactile appendages, and is freely exposed through their parietes to the surrounding medium, it is the blood alone which is trans- mitted to the special respiratory organs. The generative apparatus in the Nereids, as in the Cestoid Entozoa, is completely repeated in each successive segment; but in the Terricolce (Earth-worms, &c.) it is more localized, having only a single external orifice, as in the Nematoid worms ; and although both male and female organs are developed in the same individual, yet the congress of two is necessary, as in the terrestrial Gasteropods, each impregnating the other. The multiplication of parts by gemmation takes place to a great extent among the Annelida; for it is in this way that the extraordinary elongation of the body is effected, which is characteristic of many species ; the number of segments being thus augmented, from the single one which presents itself in the earliest stage of development, to four or five hundred. And there are certain species in which the body spontaneously divides itself into parts, each of which becomes a complete organism ; whilst in others, portions of the body endowed with locomotive and sensory organs, but unprovided with a nutritive apparatus, are budded-off, for the purpose of dispersing the products of the generative act, with which they are loaded. The nervous system here presents a far higher development than in the Nematoidea, as might be expected from the presence of distinct organs of sense, and of locomotive appendages ; for the double ventral cord is now studded with ganglia, disposed at regular intervals, and equal in size, thus conforming to the general similarity of the segments themselves ; whilst the cephalic ganglia exceed the rest in size, and acquire a directing power over them, in a degree proportionate to the development of the organs of special sense. — It is worthy of note, that even in this group, which, as a whole, is characterized by its locomotive activity, there is an entire order adapted to lead the sedentary life of Molluscous animals ; some of them even forming shelly tubes, which can scarcely be distin- guished from those of certain Gasteropods. Various modifications of I V GENERAL VIEW OF ANIMAL KINGDOM. — MYRIAPODA. 63 structure are required for this purpose, especially the concentration of the respiratory apparatus about the head ; but it is to be remarked, that these special modifications begin to make their appearance at an advanced stage of development, — these Tubicolce, up to a certain point, not only leading the errant lives of the Nereids, but exhibiting an almost exact conformity to their type of conformation. 48. Disregarding the more aberrant forms of the Articulated sub- Jdngdom, and restricting ourselves to that tolerably regular series which will best illustrate the principle of progressive differentiation, we now come to the class Myriapoda, in which we find the regular evolution of this type attaining its maximum. The segmentation of the body in this class is rendered very distinct, by the hardening of the integument of its successive divisions, and by the interposition of a flexible membrane be- tween each pair, so as to allow of considerable freedom of motion ; and the same kind of articulation is presented also by the locomotive appendages. These in the lulus (Fig. 54) are very numerous, like the segments to which they are attached, but are very imperfectly developed, showing only a slight advance upon the ventral setse of the Nereids, which they may be considered to represent ; so that the animal seems rather Fig. 54. ,7Mfr 'SAT -383* lulus. Fig. 55. \\ p » 1 A i*. 1 Ml ■*,'] ■vl ■ J L v- *v;i I /'■',' Sfr Bf to u 11 W im: 7 V .1 {'.rill UB ■I,, ■; f." m .", jtf r m Scolopendra. ide or crawl with their assistance, like a Serpent or Worm, than to rely on them for support and propulsion. The case is different, however, with the Scolopendra (Fig. 55); for here the number of segments is 64 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. greatly reduced, whilst each attains a higher grade of development, especially as regards its locomotive appendages and the muscles by which they are put in action. Still we observe as complete an outward equality in the successive segments, as the Annelida present ; it being the head alone which presents a marked differentiation in the lulidce; whilst the appendages of the first segment of the body in the Scolopendridce, instead of being employed for locomotion, are so modified as to serve to hold and tear their prey, and are also provided with an apparatus for instilling poison into the wounds they make. — The alimentary canal of the Myriapods for the most part exhibits a division into oesophagus, stomach, and intestine, the stomach usually possessing distinct muscular walls, and being sometimes lined with a plaited horny membrane, so as to constitute a kind of gizzard ; and long tubular cseca are connected with various parts of this canal, which probably act as salivary, hepatic, and urinary glands. The long vascular trunk, or ( dorsal vessel,' exhibits a regular segmental division, each portion being in fact the impelling cavity or heart of the segment in which it lies, whilst there is such a communication between the several chambers, that a general movement of the blood from behind forwards can take place through the trunk formed by their union. The venous circulation, as in Insects, Crustacea, and Arachnida, is carried on through the general cavity of the body and the interstitial lacunae in the members. The respiratory organs, which are here internal, are repeated with almost perfect uniformity through the entire series of seg- ments ; each having its pair of stigmata or breathing pores, which lead either to simple air-sacs, or to branching tracheae. Both in lulidce and Scolopendridce, the sexes are distinct ; but the generative apparatus of each sex is still extended through a considerable part of the body, although it has but a single external orifice. In the development of the organism, we still witness a multiplication of segments by gemmation; one after another being produced, subsequently to the young Myriapod's emersion from the egg, until the number proper to the species is attained. The nervous system is formed upon precisely the same general plan as that of the Annelida; the ventral ganglia, however, being considerably larger in proportion, in accordance with the higher development of the locomo- tive apparatus ; whilst the cephalic ganglia also show a great increase, in accordance with the increased elaborateness of the sensory organs. We now find, moreover, a special division of the nervous system, appro- priated to the respiratory apparatus, its ganglia being repeated, like the organs it supplies, in each segment ; and a like special arrangement of ganglia and nerves (of which traces are discoverable among the Annelida) is provided for the supply of the buccal apparatus and stomach, and is hence termed the ' stomato-gastric' system. — Thus we see that in this interesting group of animals, which exhibits in its general organization the greatest elaborateness that is compatible with the external mainten- ance of the uniform Articulate type, a certain amount of differentiation has already begun to show itself in the disposition of the internal organs. 49. This differentiation is carried to a far higher extent, however, in the class of Insects ; in which segmental uniformity is completely sacri- ficed, for the attainment of the special objects contemplated in the organization of this type. In many larvae, it is true, that uniformity is as perfect as in the Nematoid Worm or the Nereid ; but in the course of -.-..,----- GENERAL VIEW OF ANIMAL KINGDOM. — INSECTS. 65 that development which is known by the term metamorphosis, both the external configuration an d the internal structure of the several segments become more and more diversified; and at last we find the entire body separated by well-marked divisions into head, thorax, and abdomen, the thorax beino- always composed of three segments, and the abdomen of nine unless one or two of the terminal segments should have been sup- pressed. Now although all these segments, in the larva state, may have been equally provided with locomotive appendages, or may (on the other hand) have been entirely destitute of them, we find that, in the perfect Insect or imago, only the three thoracic segments are thus endowed ; this limitation of the motor organs, however, being accompanied with a much higher development of the members themselves. Each of the three thoracic segments is provided with a pair of articulated legs ; and whilst in the Myriapoda the successive j oints were almost exact repetitions of each other, we now distinguish the diversely-formed parts which are known as the coxa or < hip', the femur or < thigh', the tibia or < shank', and the tarsus or ' foot',— names which, suggested by the analogy of animals constructed upon a plan essentially different, are by no means strictly applicable. But besides these members, which may be consi- dered as homologous with the cirrhi of the ' ventral oars' of the Nereids (Fig. 53, d, e), the second and third segments of the thorax are each fur- nished (in the typical Insect) with a pair of wings, which may be likened to the membranous lobes of the ' dorsal oars' (b), being expansions of the outer tegumentary membrane over a frame-work supplied by the dermal skeleton. This skeleton often undergoes very remarkable modifications ; one piece (usually the first segment) being sometimes enormously deve- loped at the expense of the other two, so as even entirely to conceal them on the dorsal surface ; whilst in other instances a partial or complete adhesion takes place between the several rings. In either case, a degree of consolidation of the thoracic segments is afforded, which, whilst entirely I r Fig. 56. Section of the trunk of Melolontha vulgaris (Cockchafer) , showing the complexity of the muscular system. The first segment of the thorax (2) is chiefly occupied by the muscles of the head and by those of the first pair of legs. The second and third segments (3 and 4) contain he very large muscles of the wings, and those of the two other pairs ot legs. The chief muscles of the abdomen are the long dorsal and abdominal recti, which move the several segments one upon another. F w* 66 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. destroying their mobility one upon the other, gives a far more secure attachment to the complex assemblage of muscles (occupying almost the whole interior of the thorax, Fig. 56) provided for the movement of the legs and wings, to which the locomotive function is now delegated. The abdominal segments, however, for the most part preserve their primi- tive ring-like simplicity, and are put in motion, one upon another, by longitudinal and transverse muscles that differ little from those common to Articulata generally ; but the abdomen also contains special groups of muscles, developed in connection with organs peculiar to certain tribes of Insects, as stings, ovipositors, &c. All this differentiation of the mus- cular system takes place gradually, like the evolution of the organs to which the several groups of muscles are subservient. 50. The head of the Insect is not only more completely separated from the trunk than it is in any other Articulated animals, so as to be endowed with greater freedom of motion ; but it is also provided with a far more elaborate apparatus for sensation. Thus the organs of vision appear to be no less efficient in guiding the movements of these animals, than are the most perfect eyes of Yertebrata, although constructed upon a very different type : for in the place of a single movable eye on each side of the head, we find an assemblage of cylindrical or conical ocelli, sometimes to the number of several thousand, each of them adapted to receive rays in one direction only. This arrangement is common to the whole Articulated series, with the exception of the Arachnida, and is conform- able to that general plan of repetition of similar parts, of which we have seen so many examples in them; but it is in Insects thatwe find the ocelli most numerous, and their structure most complete, touch are also multiplied ; for besides the antennce, which serve by their elongation (which is frequently most extraordinary) to take cognizance of objects at some distance, we find two pairs of short palpi, whose special function it seems to be to examine substances in the immediate neigh- bourhood of the mouth ; and there is reason to believe that one or both of them are specially endowed as instruments of smell. It is certain, too, that there is some provision for the sense of hearing ; although its special instrument has not yet been positively made out. The nervous centres, with which locomotive and sensory organs are in connection, exhibit very marked characters of differentiation, in accordance with the foregoing special developments. Thus whilst in the Larva, the ganglia of the successive segments correspond in size, and are disposed at equal dis- tances from each other, we find in the Imago an extraordinary development of the thoracic centres (Fig. 57, I), which, being also approximated more closely, sometimes even run together into a single ganglionic mass; whilst some of the abdominal ganglia (m) almost disappear, and their distance remains undiminished. So, again, the cephalic ganglia (k), which in the Larva (as in the lower Annelida) scarcely surpass the ventral ganglia in size attain an enormously-increased development in the Imago ; this being obviously related in great part to the perfection and activity of the visual organs, by whose agency the movements of these animals are guided. 51. In the structure of the Nutritive apparatus, and especially of the Digestive system, the principle of differentiation is very strongly marked ; and this, not only in the type common to the class taken as a whole, but also in the subordinate modifications which its various subdivisions pre- The organs of ~ GENERAL VIEW OF ANIMAL KINGDOM. INSECTS. 67 sent, adapted as they are to exist upon the most diverse kinds of nutri- ment. The buccal apparatus presents two principal forms, one constructed for mastication (as is characteristically seen in the Beetle tribe), the other for suction (of which the Butterflies, &c. present the best examples) ; yet although these forms are subject to almost endless modifications, the same elementary parts may be everywhere traced, thus showing a most remarkable example of ' unity of type.' The alimentary canal always exhibits a well marked distinction into oesophagus, stomach, and intestine (c, d, e) ; and the stomach is frequently furnished with an apparatus suited for the mechanical reduction of the food. The glandular appendages are usually more highly developed in this class, though still preserving a very simple type, so that their character is chiefly determined by the part of the canal into which they discharge their product. The circulating apparatus is formed upon the incomplete type already noticed in the Myriapoda • for its centre consists of a many-chambered dorsal vessel (a, a), from which arterial trunks proceed to the system generally ; whilst the return of the blood takes place through the general cavity of the body, the insterstitial lacunae between the muscles, &c. Some differentia- tion is observable, however, even here ; for whilst in the larva, as in the lower Articulata, the chambers of the dorsal vessel correspond in number and position with the segments of the body, their number is reduced in the perfect Insect, by the contraction of the thoracic chambers into a mere arterial trunk, whilst those of the abdomen are enlarged and be- come more muscular ; and although each of the latter continues to act as the heart of its own segment, yet by their successive contractions from behind forwards, they propel a more vigorous current towards the ante- rior part of the body. The respiratory apparatus of Insects is very greatly extended, the trachese being prolonged into every part of the body ; in its grade of development, however, no decided advance is made upon the type of the Myriapoda ; although indications of differentiation are seen in the closure of the spiracles of certain segments, whilst those of other segments are enlarged, so that the whole apparatus is supplied with air through a comparatively small number of openings. — The repro- duction of Insects, with only one known exception, is accomplished solely by the true generative process. The sexes are distinct throughout the class ; and the males and females are frequently distinguished by diver- sities in size, configuration, or colour, as well as by the difference m their generative organs. The seminiferous or oviferous tubes possess but a single outlet, although they are frequently greatly multiplied, or, if few in number, are extended through a large part of the body ; and the last segment of the abdomen is usually adapted in the male to serve as a penis or intromittent organ, whilst in the female it is developed into an ovi- positor. In the exception above alluded to, that of the Aphides, a suc- cession of ' zooids' is produced, without any sexual intervention, by a process which seems to be essentially one of internal gemmation. No multiplication of segments by gemmation ever seems to occur in this class; the embryo, when first its outlines can be discerned, presenting the full number. It seems obvious, then, that the productive energy is here expended, not upon the ' vegetative repetition' of similar parts, but upon that higher development of a smaller number, which renders them capable of a far greater variety, as well as of greater energy, of functional e 2 I *« 68 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. power. — The general arrangement of the Nutritive organs in this class, is shown in the accompanying section of a Lepidopterous insect. As com- Fig. 57 Ideal Section of Sphinx ligustri (Privet Hawk-moth), to show the relative position of its organs ; — 1 — 13, successive segments ; a, a, dorsal vessel ; b, ligamentous bands holding it in place : c, oesophagus ; c', crop ; d, stomach ; e, e, intestinal canal ; f, f, biliary tubuli ; g, caecum j h, cloaca ; i, ovary ; k, cephalic ganglia ; I, thoracic ganglia ; m 9 first abdominal ganglion. pared with Vertebrata, the whole body may be considered as inverted; the ganglionic column which answers to the spinal cord being situated on the ventral aspect ; whilst the central organ of the circulation is on the dorsal. 52. A very analogous description might be given of the class of Crustacea, in which we find the higher type of articulated structure modified for inhabiting water; but it will be sufficient for our present purpose to notice some of the most striking contrasts which it presents between the lowest and the highest degrees of differentiation. Among some of the Isopod Crustacea, there is as complete an equality between the different segments, as there is in the Myriapoda; whilst in the Brachyourous Decapods (Crab tribe) there is a most remarkable differen- tiation, the segments of the cephalo-thorax being immensely enlarged, and fused (as it were) together, whilst those of the abdomen are scarcely at all developed, and the whole being covered-in by a carapace formed by an extraordinary backward extension of one of the cephalic rings. The same kind of alteration extends to the internal organs; for every- where we notice a remarkable fusion and concentration of what are elsewhere separate parts. Thus of the dorsal vessel, only a single chamber is developed into a heart, but this attains a very large size (Fig. 58, i) 9 and has powerful muscular walls, whilst the anterior and posterior portions dwindle- down into arterial trunks (a, k). The nervous system exhibits a like concentration; all the ganglionic centres proper to the thoracico-abdominal segments being fused into one mass, from which nerves radiate to the limbs and to the rudimentary abdomen. It is not in this manner alone, however, that the ordinary articulated type under- goes modification in the higher Crustacea; for we find the liver (e), formed rather, upon the plan of Mollusks than upon that of Insects, being a compact organ, composed of multitudes of short follicles crowded upon excretory ducts like grapes upon a stalk; and this peculiarity extends to other glandular organs, as also to the generative. All these modifications are of special interest, when taken in connection with the w % GENERAL VIEW OF ANIMAL KINGDOM. — CRUSTACEA. 69 aquatic respiration and comparatively inert habits of these Crustac ea, which mark their differentiation from the ever-active aerial Insects, as being Fig. 58. Anatomy of Cancer papmu (Common Crab) , the greater part of ^^X^/Tntestine" removed j— a. onhthalmic artery ; 6, nmscles of the stomach ; e, stomach , d, intestine^. of Cancer pagwus (Common urao , tne grea^r v *™ ui ^ -^X^ In Wine • removed ;-a, ophthafmfc artery ; b, muscles of the stomach , c, stomach d, ntetae e liver- f testis- a, flabella; h, branchiae in their normal position; t, heart; k, aDaommai artery ;' {'vault of the flank, enclosing the branchial cavity ; m, branchue turned back. I (so to speak) in the Molluscan direction. It is peculiarly interesting to observe^ that this extreme departure from the < archetype conformation only arises gradually; the young Crab, soon after its emersion from the egg, resembling the young of many long-tailed Crustacea; and the extra- ordinary development of the cephalo-thorax, with other specialities of structure, being only attained after a succession of changes that may be considered as amounting to a metamorphosis (Fig. 77). 53. Although a powerful masticatory apparatus is usually provided m the higher forms of this class, consisting of several pairs of jaws opening laterally, yet these jaws, which stand in the same relation to the cephalic segments that the legs do to the thoracic, are not completely differen- tiated from the locomotive apparatus : the three pairs which are pos- terior to the mouth presenting a gradual approximation to the anterior pairs of thoracic members, so as to be actually employed m the Stoma- pods for prehension and locomotion, whilst m the Decapods the anterior thoracic members are converted into supplementary feet-jaws ; so that a reonlar gradation is established between the typical mandibles in front ofthe mouth, and the typical legs belonging to the posterior segments of the thorax. Now in the curious Limulus (king-crab), which forms a connecting link between the higher Crustacea and the inferior group of Entomostraca, the two functions of mastication and locomotion are actually performed by the same members; for the ordinary cephalic instruments of mastication are wanting, and the thoracic limbs are so arranged round the mouth, that their basal joints may work against ~ 70 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. those of the opposite side, and may thus serve as jaws, whilst the remainder of each member acts as an ordinary instrument of locomotion and prehension (Fig. 59), In the Untomostraca, again, we find the organs of respiration very imperfectly differentiated from those of locomotion; Fig. 59. Fig. 60. Inferior surface of Limulus : — a, first pair of legs, used as prehensile organs ; b, basal joints of five posterior pairs of legs, used as jaws ; c, abdominal appendages bearing bran- chiae ; d, ensiform appendage. - A, female of Cyclops quadricomis : — a, body ; b, tail ; e, antenna ; d, antennule ; e t feet ; f, plumose seta? of tail : — b, tail, with external egg-sacs r — c, d, e, f, g, successive stages of development of young. some of the group being furnished with c fin-feet/ which are obviously adapted to serve both purposes at once \ and others having more special organs of aeration in the plumose tufts attached to the legs, jaws, and tail, as in Cyclops (Fig. 60). — There are certain Crustacea, indeed, which seem referrible as regards their general configuration to a higher type but in which there is so little specialization of parts, that no distinct provision for respiration can be made out; and in these, moreover, the sanguiferous system is reduced to its zero of development, there being no dorsal vessel, and the flux and reflux of chylaqueous fluid in the jeneral cavity being the only provision for conveying nutriment through the body (Fig. 105). This condition is in striking contrast with the ely-ramifying vascular system, and elaborately mmu constructed a striking feature of imperfection in its organs of circulation, the san- guiferous current having still to pass through the general cavity of the body and the interstitial lacunae of the limbs, in its transit from the ultimate ramifications of the arteries to the gills or to the heart 54. In the class of Arachnida (Spiders. Smrnirms &rA wp. find tiio GENERAL VIEW OF ANIMAL KINGDOM. — ARACHNIDA. 71 higher Articulated type presenting itself under another set of special modifications. These animals are adapted to breathe air like ^ Insects but by means of an apparatus of a very different kind ; and they are destined to pass their lives under very different conditions, ^^f possessing wings and spending a large part of their time m active move- ment they lurk for the most part in holes and hidmg-p aces, and obtain their'food (which is generally derived from living animals) by stratagem. The head and thorax are fused (as it were) into one mass ^ <^±" thorax • and to this all the members are attached. The tactile appen- dages possessed b Y Insects and Crustacea do not here present themse ves; t!H^Llve wholly wanting, and the maxillary palpi are deve- loped either into, instruments of prehension, or into member re* une onuraAuo ^*. Of true legs there are a Ws foui -pair constitutes the most constant external character ofthe group the thoracic limbs. The tegu- ::Z:^7^Z::Z;cZ^erMj as to its degree of firmness, in the SrS members of the class ; in the Spider tribe, which may be regarded as its Weal group, it is so soft throughout the abdominal region, that all apSSrance of segmentation is wanting; and although somewhat femerTn the cephalo-thorax, it does not serve, like the dense external skeleton of Insects and Crustacea, to give an unyielding support to the locomotive apparatus. It is interesting thus to observe the partial dis- appearance of one of the most characteristic features of Articulate structure, in a group which must be regarded as standing near the borders of its sub -kingdom. Another interesting modification is pre- sented by the structure of the eyes : which, although more ^merous than in Vertebrata (Spiders and Scorpions having six, eignt , or even more), are formed upon the plan of the visual organs m that sub- kingdom. In the structure of the respiratory organs of the higher Arachnida, again, we find a decided approximation to the V ertebrated type, although they are still multiple, and open by special orifices m the segments in which they are situated, as in Insects.— -Like the class 01 Crustacea, this group includes a large assemblage of forms which, while they correspond in general plan of organisation, differ widely m grade ol development. Thus, in the Acarida, it seems doubtful whether there is any proper circulating system; in the parasitic species generally, no special respiratory apparatus can be discovered ; and m the lowest lorms of this tribe, the sexes appear to be united, the ova being dispersed through the tissues of the body, instead of being developed wxtlun a definite ovarium. It is remarkable that, throughout this class, the young come forth from the egg in their complete form, or yearly so ; and that even in the earlier stages of their development the Arach- nidan type is very distinctly differentiated from that of other Articu- lata, so that the embryo cann ot be likened to any of the lower forms of that series. 55. We now arrive at the sub-kingdom Vertebrata, which unques- tionably ranks as the highest division of the Animal Kingdom, since it contains those classes which display the greatest perfection of organised structure, as manifested in the special adaptation of each part of their organism to some different purpose, and in the consequent number and variety of their faculties. Whilst the apparatus of Nutritive life predo- minates in the Mollusca (the sensori-motor organs being only enough de- ■ : ■P ■■■mvi 72 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. Man veloped to enable the animal to seek its food, or to meet with an individual of the opposite sex, where the union of two is required for the purpose of generation) , and whilst the activity of the Sensori-motor apparatus is the special characteristic of the Articulata, (whose actions seem to be almost entirely instinctive, that is, to be performed without any discernment or choice on their own parts), the Yertebrata are distinguished by the high evolution of that portion of the Nervous System, — the cerebrum and its dependencies, — which seems to minister to the operations of intelligence, wherein means are adapted to ends by the purpose and design of the individual itself. This attribute reaches its highest development ir whose entire organization is brought into subservience to it ; but through- out almost the entire Vertebrate series, we see a decided tendency towards it ; indications of reasoning power, and of the faculty of profiting by experience, being discernible even in the lower members of it. The pre- dominance of the Nervous System is manifested, not only in the increased size of its centres, but also in the special provision which we here find, for the protection of these from injury. In the Invert ebrated classes, with scarcely any exceptions, wherever the nervous system is enclosed in any protective envelope, that envelope serves equally for the protection of the whole body; this is the case, for example, in regard to the spiny integu- ment of the Star-fish, the shell of the Mollusca, and the firm j ointed rings of the Insect. Where exceptions do present themselves, they are in tribes which are near the higher borders of their respective sub-kino-doms so as to abut (as it were) on the Vertebrate region ; thus in the highest Crustacea, there is a set of internal projections from the shell, on each side of the median line, which form a series of arches partly enclosing the ventral cord; and in the naked Cephalopoda, the nervous centres are supported, and in part protected, by cartilaginous plates, which are evi- dently the rudiments of a true internal skeleton. It is by the high Neuro which not only encloses the nervous centres, but affords protection to the most important viscera, furnishes the system of levers required for the apparatus of locomotion, and affords fixed points for the attachment of the muscles whereby these levers are put in action, — that this sub-kingdom is most readily characterized. This neuro-skeleton, under all its special modifica- tions, consists of a longitudinal series of vertebrce; each vertebra being composed of a centrum, a neural arch enclosing the nervous centres a hcemal arch enclosing the centres of the circulation, and lateral pro- cesses ; to which are frequently added diverging appendages. The several vertebrae may be very unequally developed, both as to size and differentia- tion of parts ; thus we usually find those of the cranium (which never exceed four in number) much larger, and their components more elabo- rated, than are those of the caudal region, whose number is much less restricted. So, again, one or more of these components may be sup- pressed, the centrum being the element most uniformly present : or, on the other hand, any one or more of them may be extraordinarily developed. Thus in the cranium, especially of those higher Vertebrata whose cerebrum is large, the neural arches (Fig. 61, l ~ 15 ) are enor- mously expanded for its reception ; whilst the haemal arches of the three anterior of these vertebra (*>-**) forming the bones of the face, are also considerably extended, and their elements remarkably modified GENEBAL VIEW OF ANIMAL KINGDOM VERTEBRATA. 73 form. In the in trunk, on the other hand, the neural arches are reduced to the dimensions of the spinal cord ; whilst the haemal are expanded, chiefly by the elongation of the lateral processes which are incorpo- rated with them, so as to surround and protect the visceral mass. In front of this cavity, we find the haemal portion of the occipital ver- tebra developed into the scapular arch ( 50 - 52 ); whilst be- hind it, the pelvic arch (62—63) jg formed by a like develop- ment of the haemal portion of one of the sacral vertebrae ; and it is in the enormous development of the diverging appen- dages ( 53 ~~ 65 ) of these vertebrae, that the anterior and poste- rior members origi- nate. In the caudal region, the periphe- ral portions of the vertebrae are gradu- ally suppressed; un- til at last nothing remains but the cen- trum.* * The views of Profes- sor Owen, as set forth in his masterly works on the " Homologies of theVer- tebrated Skeleton," and on the " Nature of Limbs," as also in his u Lectures on Compara- tive Anatomy," vol. ii. ? are here followed. ^7-* — ' * A % % \ cA ?t5 o3 a -e o o co o3 © ©. q? © V) © o CO o3 P ■m P ^r,3^ & d o3 9, o r3 *r o 1 3 O co CO o3 ..... o3 © cd ^_J<» °^£ § . W ^rH ~9 S^pd P cw cv •*> P w H P — ® a ft=~ P 2 * >A $ * 1 S ""3 5 co c6 cS ° a « 2 d rH 3 ^ H P O Q o r^P W Qj CO 9 O -M *m 03 4-J rd ft £ &D. rd d^=J-^ ■a o £.3 fl 5P 2rd J-S'S.S q ft 0) CO CO S3 § flOncS 2 C* -+^> CO © a CO O g +=> "^ rd CO o^r ^P ^ &i 03 tf2 r* ^ c6 d S 5 S"* 3 &c£ ^^5 ^^P 0>r^ ftco-^ c s CO C*- fl *> CD ^Nviaa^ft o ce^^tC CO P. cm 03 ftfl d ^ CO o3 *U t .rd o3 ■M O 03 CO * O rS o3 o co o3 _r rr r o o P co P^ftH 5 P dr^ fe °rd d o3 r-t CO © *x 3 6plS ®I>C2 ® ''ro (V) Jr co h? _y ^a CO ft^ S3 "° ■% S .3 ^d 6 "^ -M CO ft ca CO o3 ftj.y o3 ^co-a ^ 0/3 cK^.-e c3^ .^-^ft^d d ~£-3 ^ ce en =s | " fe |^ |§ H "*1^ Scordio « d ftfe ^aaaiyirs ^rH^.-cq^ a> &£ £ ^ © r?H 5 d ■-p o3 „r &J d © CD CD a -a 03 ^ M o3 ■** tJDp »-• d ° - be ^r p „r a ^d p CD ^5 B^^ ^3 r d_d 4S « ft p .o d +» 5 d . « S "rt "P «H rrd CD CO sg a H P •» o ^ ^°-P CD^^'dr-T «v?5 a GQ O "5 ^ -3 «? &S 9B .-S 2 * ^> O O c *t^ & © *o v>>*r iQ r^ ^ JO pH o O oo CO o^ .v ••* -M ^ CO •n CM CD f73rd &J3 03. o3 CO ^ CD CO CD CO 03 bD ^p ^ fl r^ NO CD a rd P k"a 03 cm ftd rC5 d ^1^^ 03rd I d P P g P ft-grS CO ft ftft k>ft ftft CD P {^ P ft H r, 0) bo o3 gtfqTrd rt'ESo I 'a'Sc^ CO a CD •• _l CD d co a d * ns 5 '^r2 .f,i_>_»-03o3rt ce « d 03 .a £ -,5 85^ 5 oq ffi § I 1. CO CO CO CO CO © SrO P ^ © ©"H O CO^^^rd * E » oa a ftn,2 ".a S'rt oa a 9<& ftftP S.^S^-fab Rd 03 -M. I 1 ^ ^ O ^ r^J^ c8 © o © a aa r-( b£ 03 ?,.^ a %£ sb a.>: a SSSSSAa.afB ft 1 cd CO b£ •. r^ rH^ O C3 rH c3 rH c3 ^ cd PH M pM pH CO 4-J 03 cl'S © ph © © a 03 ft "rQ^S CD -2 •» rS M _» ^j »0 sJ ©-M CD CD "3 SB pH ^ ® rH 1 ■ * CO ft e c3?i ill s il'll irnn 1L 74 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 56. The Vertebral skeleton, as a whole, is very differently developed in rious parts of the series. Thus in the < apodal' Fishes, locomotion is per- various Worm find the skeleton extremely flexible, and its divisions indistinct, the spinal column being a continuous cylinder of cartilage with scarcely a trace of segmentation. In the Serpent tribe, again, which is destitute of loco- motive appendages, the vertebrae are extraordinarily multiplied, and are so connected by ball-and-socket j oints, that the entire column possesses a most extraordinary degree of flexibility. In proportion, however, as distinct members are developed, and the power of locomotion is committed to them, we find the firmness of the spinal column increasing, and its flexi- ■in which, as in Insects, the movements of mini the body through the air are effected by muscles that must have very firm points of support — the vertebral column is much consolidated by the union of its different parts, so as to form a compact franiework. As a general rule, then, the mobility of the extremities, and the firmness of the vertebral column, vary in a converse proportion. The number of these extremities in Vertebrata, never exceeds four ; and two of them are not unfrequently absent. The power of locomotion is not developed to nearly the same proportional extent as in the Articulata ; the swiftest Bird, for example, not passing through nearly so many times its own length in the same period, as a large proportion of the Insect tribes : but it is far greater than that which is characteristic of the Mollusca ; and there is no species that is fixed to one spot, without the power of changing its place. On " *_hest Mollusca approach them very nearly in the development of organs of special sense, of which Vertebrata almost invariably possess all four kinds — sight, hearing, smell, and taste. 57.^ A like union of the characters of the Articulated and Molluscous sub -kingdoms, may be noticed in the general relations which the organs of Nutritive and those of Animal life bear to one another. The former, contained in the cavities of the trunk, are highly developed ; but, as in Mollusca Of the latter, the nervous system and organs of the senses occupy the head, whilst the muscles of locomotion are principally connected with the extremities ; both are symmetrical, as in the Articulata ; but, whilst that part of the nervous centres, which is the instrument of reason, is very largely developed, the portion which is specially destined to locomotion, together with the muscular system itself, bears much the same proportion to the whole bulk of the body, as it does in the Articulated series. Hence we observe that the Vertebrata unite the unsymmetrical apparatus of nutrition, characteristic of the Mollusca, with the symmetrical system of nerves and muscles of locomotion, which is the prominent characteristic of the Articulata ; both, however, being rendered subordinate to the great purpose to be attained in their fabric — the development of an organiza- tion through which intelligence may peculiarly manifest itself. For the operation of this, a degree of general perfection is required, which is not manifested elsewhere ; and, in order that the body may be always in a state ^ of readiness for active exertion, it is requisite, not merely that it should be adequately nourished, but that it should be constantly maintained at a high temperature. This is fully the case, however, with only the two higher classes of Vertebrata ; which, in their power of o-ene- GENERAL VIEW OF ANIMAL KINGDOM.— VERTEBRATA. 75 ratin sufficient heat to counteract the effect of external vicissitudes, and the Invertebrated classes. uie utwwui. . Indications of a like power, however, though tea energetic In its operation, are presented in Reptiles and Fishes. The maintenance of a constantly-high temperature, and the support of the svstem under the demands created by its unceasing activity involve an energetic performance of the functions of respiration and circulation j and thesl again require a constant supply of alimentary material and great activitv in the process of digestion. The Digestive apparatus usually presents a high degree of specialization; its simplest forms (as among the Invertebrata) being found in those tribes which obtain their food by the suction oknimal juices, ^^J^^^Z^^^r feeders. valvular tube but it is completely differentiated from them mfunction also; the iStine itself genLlly exhibits varieties of conformation and of endow- ments in different parts of its course j and the number and comptoty of dands is greatly augmented. The anterior part (considering- accessory cavi tilt/ OllillllCti- CLO pXC^V-'V^Vi. ixv^AMv-v. j j • "I 1 Jl • pied by the heart and respiratory organs, and the posterior by the diges- tive and excretory apparatus ; it is only in Mammals, however, that the separation between these portions is completely made, by the interposition Fig. 62. o -.*■ 7 -n.^+voHTio- tVm Vertebrate type of structure : — P, pectoral ex- Ideal Section of a Mcmmal, Mnstrzti ng the ^tfraieyp ^ ^Tfj '' „7aXtm • % tr ep^ ottisT'^tomach ; I/heart ; i, intesfine -j, J, Jaws ; '* kidney ; f^.Vtt'^iSi c'/cesophagus ; ol, organ of smell; op organ of vision , o's montfi' containing organ of taste; p, pancreas; 8, spleen.; sfc, sk, skeleton; *, trachea; , ' i.-ii- ... „^„o™ Harirlw • «. «. vertebral column. of a diaphragm. The general cavity of the body is now altogether cut-off from participation in the transmission of nutritive fluid ; a special system of vessels being interposed for the absorption of sanguifying materials, alike from the digestive cavity, and from the interstitial lacunae of the tissues generally, and for the introduction of these into the blood-current ; whilst the proper sanguiferous system of vessels forms a completely-closed circuit. All Vertebrata (save the Amphioxus) have red blood, which is ^^B 76 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. put in motion by a single distinct muscular organ, the Heart • and the Respiratory organs, whether formed for aquatic or for atmospheric respira- tion, are always placed in immediate connection with that organ so as to receive a stream of blood directly from it. The respiratory apparatus moreover, always draws-in its supplies of the aerating medium, whether air or water, through the mouth ; and these supplies are renewed by means of rhythmical muscular movements, sustained by an automatic action of the nervous system. The rest of the apparatus for the depura- tion of the blood presents a very high development in the Vertebrated classes ; the structure of the glands by which it is effected being such, as to combine the greatest functional activity with the closest concentration ot the organic mechanism concerned in it. 58. The power of Reproduction by gemmation is limited in this series to the reparation of lost or injured parts, and nowhere extends (save at an early period of the evolution of the germ, when its grade of development corresponds with that of the simpler Zoophytes*) to the multiplication of independent 'zooids.' The two kinds of generative organs being never combined m the same individual, the concurrence of two of opposite sexes is required for each generative act. In the lower Vertebrata, it is sufficient ■that the products of the male and female organs should come into contact alter their extrusion from the body; but in the higher, the ova are fertilized while yet within the body of the female, and are commonly retained there for a shorter or longer time afterwards. In nearly all V ertebrata, the young animal, when it first comes forth into the world has the characters of the class to which it belongs ; and those of its order family, genus, and species, if not at once distinguishable, speedily become so, the special type being evolved out of one more general, without any true metamorphosis : such a metamorphosis, however, does take place in one remarkable group, that of A mphibia or Batrachian Reptiles : the members of which come forth from the egg in the form and condition of Wishes, and gradually assume that of Reptiles by a series of changes in which every part of their organism is concerned. 59 The Vertebrate type of structure displays itself under four principal aspects; m comparing which we can scarcely fail to recognise the difference so frequently insisted-on, between grade and plan of development. For the Physiologist has no hesitation in affirming, that, whilst each of the classes of Fishes, Reptiles, Birds, and Mammals, has a certain charac- teristic conformation that is typical of it, they may be regarded as parts of a series, which on the whole ascends with considerable regularity from the lowest Fish to the highest Mammal ; since he traces in every one of the chief divisions of the organism, alike in the apparatus of animal and m that of vegetative life, a gradual progress from a simpler and more general to a more elaborate and specialized type. This is peculiarlv obvious m the organs of Circulation and Respiration, and in those of Re- production; and it is from these, accordingly, that the Naturalist draws his best characters for the separation of the four classes in question. Ihus m Fishes, which are specially adapted for an aquatic life, the the^relt?? ^^ th , iS ex fP tiw ? wiU b e hereafter shown (chap, xi.) to be required by tue present state of our knowledge in regard to tbe formation of double monsters. GENERAL VIEW OF ANIMAL KINGDOM. — FISHES. 77 Mollusca aeration of the blood is accomplished by ^ heart (as in that sub-kingdom) possesses only a single auricle and ventricle ; but the relation of the heart to the branchial circulation, and the mecha- nical arrangements for renewing the water in contact with the gills, are such as to afford to the blood of this class a far higher degree of oxygenation, & j*« m. «h j«fc mm -i -^ m k. H & V ■ *• *■« Mollusks The three higher classes of Vertebrata are organized for atmospheric respira- tion; but to this, in Reptiles, only a part of the blood is subjected; the heart usually possessing only three cavities, and the blood transmitted to the system being a mixture of that which has been just returned from it in a venous condition, with blood that has been arterialized by trans- mission through the lungs. On the other hand, in Birds and Mammals, the arrangement of the circulating apparatus is such, that the pulmonary and systemic circulations are completely separated, each having its own heart, and the one following upon the other ; so that the whole current of blood is alternately transmitted to the system and to the lungs, and what has returned from the tissues in a venous state is entirely oxygen- ated before being transmitted to them a second time. But these two classes, whilst separated from Fishes and Reptiles by the type of circu- lating and respirating apparatus which they both present, are separated from each other by the mode in which their generative function is performed; Birds, like Fishes and Reptiles, being oviparous; whilst Mammals retain their ova within their bodies, until the embryo is suffi- ciently advanced in its development for it to be nourished externally by mammary suction.— As the structure and action of the principal organs of Vertebrata will hereafter be considered in detail, it will be sufficient here to indicate the general plan of conformation which prevails in each class respectively, and especially that part of it which is exhibited in the structure of the skeleton. 60. The skeleton of Fishes departs less from the ' archetype,' than does that of either of the other classes : but it presents, nevertheless, certain special modifications, that adapt it to the peculiar conditions under which these animals are destined to exist. Thus with a low deve- lopment of the neural arches of the cranial vertebrse, in conformity with the small size of the brain, we find the haemal arches of great size ; the greater part of their expansion being destined to give to the buccal apparatus the power of prehension, which is necessitated by the absence of prehensile power in the limbs ; whilst certain of their elements are specially adapted to afford support and protection to the respiratory apparatus. So in the conformation of the skeleton of the trunk, we find its lowness of type indicated by the ' vegetative repetition' of similar parts ; the vertebral segments differing much less from each other, than they do in any of the higher animals save Serpents, so that no distinction into regions is marked-out. The vertebrae, too, are very loosely framed together, so that great freedom of motion is permitted in a lateral direction; the support everywhere given by the medium which these animals inhabit, preventing the necessity that exists in higher animals for such a close connection of different parts of the framework, as shall enable it to rest securely on a limited number of points. The most remarkable peculiarity which this condition involves, is the cupped form ^^^^^v^v MMMBWH if 78 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. of each of the surfaces of the ' centra' or bodies of the vertebra; (Fig. 63), Fig. 63. A B A, Oblique view of the Vertebra of a Cod ; b, Section of three connected vertebrae, showing the intervertebral spaces. which work over an inter- vening series of doubly-con- vex intervertebral capsules, in such a manner as to im- part great freedom of move- ment to the entire column. It is by lateral strokes of the tail and posterior part of the trunk, whose surface is extended by the vertical median fins, that the acts of locomotion are chiefly effected; the functions of the pectoral and ventral fins, which answer to the anterior and posterior mem- bers of higher animals, being generally limited to the body and its direction. balancing changing , . ,. , x . There are certain cases, however, m which the enormous development of the pectoral fins enables them to serve as the special instruments of locomotion ; and when this is the case, the spinal column is shorter and less flexible than usual. The ' rays' upon which the median fins (dorsal, caudal, and anal) are supported belong, not to the vertebral but to the dermal skeleton ; as do also, it seems probable, the intercalary bones that support them, which are implanted between the neural spines of th e vertebra;. In the pectoral fins, which are always situated immediately behind the head, in connection with the occi- pital segment from whose haemal arch they are developed, the arm and fore- arm do not manifest themselves externally, being hidden within the trunk ■ the extended surface of these fins is consequently given by the elongation and multiplication of the carpal, metacarpal, and phalangeal bones ; and their movements are limited to flexion and extension at the wrist-joint There is no sternum in Fishes, the haamal arches of the dorsal vertebra; being never closed- in beneath; and the scapular arch is consequently destitute of the support which it elsewhere derives from that bone, beino- only completed by the meeting of the coracoids on the median line'. The ventral fins, which are formed upon the same general plan with the pectoral, have no fixed position : for the pelvic arch is not connected with the spinal column in any other way than by a ligamentous band, and there is no such consolidation of two or more vertebral centra for its support as elsewhere constitutes a < sacrum.' In those fishes espe- cially, which enjoy a considerable range of depth, the ventral fins are advanced towards the head, being sometimes situated even in front of the pectorals.— The dermal skeleton of Fishes is usually more developed than it is m the higher classes, and comes into closer connection with the neuro-skeleton. It sometimes forms a complete bony envelope to the entire body, and seems to have done so especially in the Fishes of the earlier periods of the world's history; but the general investment more ik: GENERAL VIEW OF ANIMAL KINGDOM. REPTILES. 79 commonly consists, in the existing Fishes, of thin plates, whose structure is essentially cartilaginous, though true osseous tissue is still commonly found in the dermal c rays' and intercalary bones which support the median fins. To the dermal skeleton, moreover, may be referred the Teeth ; which in this class approximate much more closely in structure to bone, than they do in Reptiles or Mammals ; and become more inti- mately connected with the bones of the jaw, so as to be even joined-on to them by continuous ossification. 61. In the Muscular and the Nervous apparatus, we observe the same tendency to vegetative repetition of similar parts, coupled with special developments for particular purposes, as we have noticed in the skeleton ; and the cephalic centres of the latter are remarkable for the very small size of the Cerebrum, in proportion to that of the sensorial centres. The Sympathetic system of nerves, moreover, is not distinctly differentiated in some of the lower Fishes from the cerebro-spinal ; and even in the higher, it retains a peculiarly intimate connection with the pneumo- gastric nerve. The generative apparatus of Fishes generally, although in some respects higher in type than that of the Invertebrata, is lower in grade of development; there is no intromittent organ, the ova being fertilized, after they have been deposited, by the casual contact of water through which the male semen has been diffused. The embryo at the time of its emersion from the egg, is far from having acquired all its parts and organs, and depends for some time upon the store of food contained in the yolk-bag which hangs down from the abdomen ; and the young receives no care or sustenance from its parents. In the higher Cartilaginous fishes, on the other hand, there is a true sexual congress, so that the ova are fertilized within the body; and they are sometimes delayed there until an advanced period of the evolution of the embryo, deriving additional supplies of nutriment from the parent during the latter part of this period. And it is worthy of special remark, that with this prolongation of the period during which the embryo is supported j from externa] sources, a much higher grade of development is ultimately J attained by many organs, especially by the nervous system. 62. The class of jReptiles includes a Collection of animals of very diver- sified conformation, which nevertheless agree in certain leading pecu- liarities of anatomical structure and physiological action, that distinguish them from all other Vertebrata. They are for the most part adapted to inhabit the surface of the earth, along which they creep or crawl, rather than run or leap ; and those which live in water are obliged (with few exceptions) to come to the surface to breathe. The arrangement of their circulating and respiratory apparatus, however, being such as to aerate only a part of the blood-current, their demand for oxygen is far less energetic than that which exists in other Vertebrata : and they can endure a longer privation of it. This want of respiratory activity is (so to speak) the manifestation of that general inertness, both of the organic and of the animal functions, which is the distinguishing physiological character of the class. Their demand for food is not frequently repeated, and they can sustain a very long privation of it ; their perceptions are obtuse, and their movements are sluggish ; altogether they may be said to live very slowly, though their degree of vital activity varies with the temperature. There is a marked general accordance, again, among all the orders of i! 80 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. Keptiles, in the degree of development of the cerebrum, as compared with the sensorial centres ; this being intermediate between the almost rudimentary condition under which that organ usually presents itself in Fishes, and the greatly-augmented size which it possesses in Birds. 63. The differences between Frogs, Serpents, Lizards, and Turtles, are most apparent in the skeleton; but these depend rather upon the suppression of certain parts, and upon the excessive development of others, than upon any essential diversity of type. The skeleton of a Lizard, at the first view, does not present any marked difference from that of a Mammal. We find an evident distinction between neck, trunk, and tail ; and both the anterior and posterior extremities now have a fixed position, and are so connected with the spinal column that the weight of the body may be supported on them. In no existing Reptiles, how- ever, are more than two vertebrae united to form a sacrum ; so that the junction of the pelvic arch with the spinal column cannot possess the same firmness as in Mammals, whose sacrum is usually composed of from four to six segments. The total number of vertebrae is often very considerable, but the multiplication is chiefly in the caudal region ; and the caudal vertebrae are remarkable for having their haemal arches con- tinued backwards like the neural, forming what are known as the chevron-bones. The most distinctive peculiarity in the skeleton of the trunk, is the backward prolongation of the sternum and of the sternal ribs over a considerable part of the abdominal region. The bones of the extremities are externally formed nearly upon the plan of those of terrestrial Mammals ; but differ from them in the inferior degree of deve- lopment of processes for muscular attach- ment, and in having no medullary cavity, Fig. 64. their interior occupied being throughout by cancellated struc- ture. Their dimen- sions are proportional to the weight which they have to sustain, and to the use that is to be made of them in the act of progression; and we may pass from the order of Lizards towards that of Ser- pents through a con- tinuous series of forms ,. , , , (Fig. 64), in which the limbs become more and more feeble until we lose all external traces of them, whilst at the same time the body becomes more elongated and serpent-like. In the true Serpents, the locomotive power is entirely withdrawn from the limbs (the rudiments of which, where they exist are applied to the purpose of < claspers' in copulation) ; and not only are the scapular and pelvic arches rudimentary, but the sternum is JBimanns . Seps. GENERAL VIEW OF ANIMAL KINGDOM. REPTILES 81 entirely undeveloped, so that the extremities of the ribs are free, as far at least as the endo-skeleton is concerned. On the other hand, the body is immensely elongated, chiefly by the multiplication of the dorsal ver- tebrae; thus in the Python, the total number of vertebrae is 422, of which about six-sevenths possess ribs. The spine is extremely flexible; and the ribs, whose extremities are connected with the abdominal scuta of the integument, can be employed in some degree as instruments of progression, moving backwards and forwards beneath the skin. In the elongated and cylindrical form of their bodies, in the multiplication of its segments, and in their mode of progression, Serpents bear a striking analogy to the Iulidse (Fig. 54). Their internal organization partakes of the general elongation ; for one of the lungs (the other being very little developed) and the ovary extend, like the intestinal tube, through a large part of the cavity of the trunk ; and the liver and kidneys are also considerably lengthened, although not to the same degree. In the structure of the nervo-muscular apparatus, there is a remarkable degree of c vegetative repetition,' as in the lower Articulata; for the spinal cord, although forming a continuous column, is of nearly the same diameter throughout, resembling in this respect the gangliated ventral cord of the Myriapods ; and the successive pairs of intervertebral nerves which it gives off, supply muscles whose form and arrangement are almost exactly the same, from one end of the body to the other. — In the Batrachian Eeptiles (Frogs, &c.) ? a modification of precisely the opposite character is superinduced upon the ordinary Reptilian conformation; for the body ^ is shortened, not merely by the non-development of the tail (at least in the Anoura, which are the types of the group), but also by a reduction of the number of vertebne of the trunk ; whilst, on the other hand, the extremities are enormously developed, locomotion being entirely performed by their agency; the ribs, moreover, are unde- veloped, but the sternum is of large size, so as to give protection to the viscera, as well as attachment to the muscles which cover them. — Another still more remarkable modification is presented in the Chelonia (Turtles, &c), which have the body enclosed within a c shell' formed of a carapace and plastron; the carapace or dorsal arch being formed by the coalescence of dermal bones with the expanded ribs ; whilst the plastron Fig. 65. Emysaura Serpentina. or ventral shield, is formed by an expansion of the sternnm and sternal ribs, still further extended by the development of portions of the dermal G ^^^ hum 82 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. skeleton in continuity with them. The spinal column is here com- pletely immovable, the vertebrae being anchylosecl to each other and to the carapace \ so that the act of locomotion is completely delegated to the extremities, the bones of which are highly developed, A transition between the Chelonia and the Sauria is established by such animals as the Emysaura (Fig. 65); which have the shell so contracted, and the neck, tail, and limbs, so much elo] within it ; and which have at least as much of the Alligator as of the Tortoise in their general habits. — Besides the existing orders of Reptiles, we are acquainted with fossil remains, which must be referred to this class, but for the reception of which three additional orders must be formed ; and it is interesting to remark that these orders may be consi- dered as representing the other three classes of Vertebrata. Thus the awn ■ Fig. 66. * Skeleton of Ickthyosawrus , Enaliosauria, of which the Ichthyosaurus (Fig. 66) and the Plesiosaurus are of Fishes; their vertebrae being biconcave as in that class, and the Fig. 67. Skeleton of Plesiomnrtis. whole skeleton of the trunk having an extraordinary mobility; but the act of locomotion having been chiefly performed by the members, which are constructed rather upon the plan of the paddles of the Cetacea (though with a 'vegetative repetition in the number of phalangeal ■ ■* * 1 GENERAL VIEW OF ANIMAL KINGDOM. REPTILES. 83 ■ bones), and are connected with the spine by scapular and pelvic arches tdZT V q G stren S th — So , again, in the Pterodactylus we find the be ft 7 t^T ^ adapted t0 a Bird " lik e life : the chief modification Kh™S? a l en ° r ^f T tieS ' wMch W a sin S le <%* enormously Decnlinri^Ao »u« • rr xi. "^S""^ expansion ^ig. lj; but various S^SJ^S.TL^S?.^ be -« "Wvablein other pa* of the skeleton, which were doubtless ncations of its internal organization. connected with analogous modi- Of these, the hollowness of the wing-bones, which so strongly re^'i^rtj^^^ ftm itTowll ° h T iT mista T k - for them, - one that is interesting rep* ent^fe^ , relatl0ns — Lasth /> the Mammalia seem to have beeS represented in the order Dmosauna, consisting of gigantic Reptiles whieb were elevated upon much longer limbs than existing Lizards I7thlr skeletons present several features of analogy to that class, the surfaces of the vertebrae being generally flattened, the ribs being connected I wUh the vertebrae by a double articulation, at least five vertebrae benig ^n ^f?ti i SaCrUm ' S ° aS *° give a Yer y firm basis to *te pelvic arch A'JS; ^ Verteb J al foments of the Reptilian cranium, although not as xn ™ ZdTd a f 6 1 " ^ m]X > Und ^° far ^- -aWnce than ra tT7^ / malS ; and the mimber of bojl es which remain sepa- of the htw 6 V6 7 C T iderable - ThuS in the Crocodi H although one arch of tSn Tf mg f ° rmS f the daSS ' the four P ieces <>f ^e leural of Ma. . Pltal Segment ' wMch coalesce t0 form the 'occipital bone' reprte^ *°> ***»> the >henoFd boneMs b/four ^ In 111 th^ 06 !' tb , 6 + f r tal . b0Iie ' ^ tW > and the temporal' ^th th e ^l be « ca ^ ^es, the occipital segment is articulated wbi^f* . e ™ column b 7 a single condyle formed b y its centrum o? he nlcSTst ^e ^ ? ^ 5^ f ' atW (« <*£S£ n rw J^- J x convexity at the back of each ordinarv vertebra kinnedll **?• * v. 00 *^ 5 the Vertebra bebind * ' - the nS- ~ ^c WW V ' ^7^' ^ co ^ ecti on is made by a double condyle, animaltf I^V^ ^T t0 disti *g^ the skull of any of these of Zt ' A S at ° f aU ^ Re P tileS - In the Perennibranchiate forms IruZf A N the S eneral conformation of the skull, as well as of other parts ot the skeleton, presents a very close approximation to the ichthvio I?U^ til 1 ° f SerpentS 1S Cbiefl ^ ^arkable for the extraoXarv dilatabihty of the entrance to the mouth • which is especial! v rl , ! / the want of union between the two halves of the lower faw t° median line, and to the mobility of the elements nf Ihl 7 J ? e bular arch with which it is articulat^ 1? !, i tympano-mandi- approximation betweer! ^ tl bone S ' o ke Z 7 ^v ?? ^^ ligamentous connection with each other 1^*%'^ **? °^ a hand, the component pieces of fV\ « he < ? d ? nM » on ^e other other; and an increased e X f^+ J " ^ ^ firm1 ^ United to e ach surface of atJZ^^^T " ^ M T\ Tu f leS to tbe of a bony arch covering its extS V T^ muscl + \ b ^ the addition J g exterior,— this, however, not being an entirelv g2 ■ I ^mm 84 GENERAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. new structure, but being formed by expansions proceeding from the parietal, posterior frontal, malar, squamosal, and mastoid bones. With the exception of the Chelonia, nearly all Reptiles possess a dental appa- ratus, which is intermediate in its structure and mode of development Mammals, For, save among a few extinct genera, all Reptiles possessing teeth exhibit more or less of approximation to the formation of sockets for their reception; and a very interesting correspondence may be traced between the various grades of such approximation, and the successive stages of dental deve- lopment in the embryo of higher animals. Thus whilst, in the extinct Mososaurus, the tooth is simply anchylosed at its base to the margin of the jaw (Fig. 68), — no dental groove having ever been formed, but only Fig. 68. (■■ • : Skull of Mososaurtcs. dental papillae, — in the greater number of ordinary Lizards and Frogs, the alveolar ridge is elevated on the outer margin of the jaw (the teeth being anchylosed to it), so as to form the outer wall of the dental groove, as in a Human embryo of about the 7th week ; in the extinct Ichthyo- saurus, this groove is completed by a corresponding elevation of the inner margin of the jaw, and an indication is presented of transverse division into sockets, as in a Human embryo somewhat more advanced; in most Serpents and some Batrachia, the teeth are surrounded by shallow sockets, to which their bases become adherent; whilst in the extinct Plesiosaurus and the whole Crocodilian group, the sockets are deep enough to give firm hold to the teeth, which remain free as in Mammals. The base of the teeth in Reptiles, however, is seldom contracted into a < fang' ; and in no instances are Reptilian teeth implanted like those of Mammals by diverging fangs. The form and structure of the teeth in this class are for the most part conformable to one simple plan, there being very little differentiation as to either ; they seldom depart much from the simple cone, more or less acute at its point, and are obviously adapted more for seizing and holding prey, than for dividing and masti- i rt »* GENERAL VIEW OF ANIMAL KINGDOM. — REPTILES. 85 • -V ft ft catmg food ; and their most remarkable modifications, one for carnivo- rous (Fig. 69), and the other for herbivorous regimen (Fig. 70), are seen Fig. 69. Portion of jaw of Megalosmvrus . Fig. 70. Portion of lower jaw and teeth of J [guano don. in two genera of the extinct order of JDinosauria, which in many other F o,x uxuuiars appear to have approached Mammalia. In the entire order of Chelonia, the teeth are replaced 'by a horny beak, which is developed, like the teeth, from a number of distinct papillae along the margins of the jaw-bones ; and the same is the case with the larvae of the Batrachia, which, however, cast off the beak when they assume the perfect Rep- tilian condition, their jaws then becoming furnished with teeth. 65. The class of Birds may be considered as the physiological antithesis to that of Reptiles ; its plan of development being far more completely opposed to that which prevails in the last-described class, than is that of Mammals; though as regards the grade of development of the greater number of their organs, Birds are manifestly intermediate between Rep- tiles and Mammals.-Taken as a whole, the members of this class exhibit a remarkable conformity to one general type, presenting in this respect a marked contrast to the diversity which we have seen to prevail amongst Reptiles; and this conformity is both structural and physiological beina shown alike m the plan of organization, and in the working of the oro-ani? mechamsn, They have been denominated, and not inappropriate^ the Insects of the Vertebrated series f possessing as they do a series of 86 GENERAL adaptations for passing their lives, not upon the solid ground, nor in water, but in the elastic and yielding air, which are very analogous (though, from the difference between the Yertebrated and Articulated types, they cannot be homologous) to those which form the essential characteristics of Insects. This is especially obvious in the extension given to the respiratory apparatus through a large part of the fabric ; which has the double effect of adding to the respiratory surface, in the degree necessary for that perfect oxygenation of the blood which is required to sustain their wonderful activity of movement, and of dimi- nishing the specific gravity of the body by the substitution of air for more weighty matters. Birds, again, resemble Insects in the complete delegation of the locomotive power to the members, and in the consoli- dation of the skeleton of the trunk ; and the movements of flight are performed in a manner essentially the same, although the wing & of the Bird has no anatomical relation to that of the Insect, being rather the representative (so far as any part of a Vertebrated animal can represent a part of an Articulated) of its anterior legs.— Birds stand in remarkable contrast with Reptiles, moreover, in vital operations are performed. The _ * * _ H & mm ^ with high degree of nervo-nmscular activity which they put forth, can only be kept-up by a very energetic performance of the nutritive processes ; and this involves, and is at the same time subservient to, the maintenance of a very elevated temperature (1 00°— 1 1 2°). Some Insects, during the period of their greatest activity, generate as much heat as Birds ; but they cannot sustain this, and are consequently liable (like cold-blooded animals generally) to be reduced to a state of complete torpidity by a depression of external temperature, which Birds have the power of resisting. In the tegumentary covering of Birds, we have a most remarkable combination of attributes ; for it serves alike to retain the animal heat within the body, in virtue of its non-conduct- ing power, and to give the required expansion of surface to the wings. It is only in a few birds which are incapable of flight, that we find any considerable modification of this important feature ; the Benguin, which has many Reptilian affinities, having the feathers on the anterior mem- L-*. *^*msmr*4 / mmmmm I^v^.--^|- k „ J__ f* • "1*1* _•■ — m\ . _ _ bers (which act as fins in propelling the bird through the water) metamorphosed into the likeness of scales ; while in the several members of the Ostrich tribe, which usually have wings still more undeveloped than those of the Benguin, and which are only fitted for progression by running like Mammals upon their elongated legs, the feathers present closer and closer approximations to hairs. 66. The bony frame- work constituting the Bird's vertebral skeleton, presents a series of peculiarities which distinguish it most completely from that of other Yertebrata ; and, as in Reptiles, there is no part more characteristic of the class, than the cranium. Whilst that of Reptiles is remarkable for a permanent separation of a great number of its verte- bral elements, that of Birds is no less remarkable for the close union which very early takes place among these, so that the sutures between the different portions of the neural arches are abolished, and the whole brain-case seems formed of a single piece. The bones of the face, how- ever, do not undergo the same consolidation ; for they are always so connected with those of the cranium, as to possess considerable mobility ; and their union among themselves is seldom complete. In the mode of GENERAL VIEW OP ANIMAL KINGDOM. — BIRDS. 87 articulation of their lower jaw, Birds agree with other Oviparous Yerte- brata, and differ from Mammals ; for the tympanic portion of the tem- P °^ al T? ! )0rLe remains distinct from the rest, and forms, as in Reptiles and Pishes, a pedicle interposed between the jaw and the solid part of the cranium. Each ramus of the j aw is originally formed, as in Rep- tiles, of six pieces; hut whilst these remain permanently distinct m Keptiles, their coalescence begins very early in Birds ; and this goes on until the number of pieces is reduced to three on either side ; whilst by the coalescence of the two which touch on the median line, the total number of pieces is further reduced to five. It is interesting to remark that the traces of the original separation of these elements remain longest m those aquatic Birds, which show in other parts of their conformation the closest approximation to the B ~ Reptiles, is still articulated with the vertebral column by a single tubercl type. The skull, as in most the vertebral the separation of the articulating surface, however, into two lateral halves being occasionally presented. — The length of the neck, and the number of vertebra? of which it is composed, vary considerably in the different tribes of birds, and seem in each to have reference to its own peculiar requirements ; thus in the long-legged Waders and Struthious birds, the neck seems elongated for the purpose of enabling the head to reach the ground without flexure of the limbs ; whilst in the Swan and other short-legged aquatic birds, we find a similar elongation, the purpose of which is obviously to allow the bill to be plunged deep into the water in search of food, whilst the body is floating on the surface. The tail, on the other hand, is always very short, or even almost rudimentary ; and it serves no other purpose than to support the tail-feathers, by whose agency the birds of active flight are steered as by a rudder. Whilst the cervical vertebrae are connected with each other in such a manner as to ensure great mobility (synovial capsules being interposed between their articulating surfaces, in the place of intervertebral fibro-cartilage), those oi the dorsal region (especially in Birds of active-flight) are so united, as to provide for the greatest fixity that is compatible with the degree of mo buity required in the osseous framework for respiratory and other purposes. The ribs have for the most part a double articulation with the spinal column ; and they are prevented from moving too freely upon each other, by the splint-like processes (' diverging appendages') which project from the posterior margin of each, and overlap the rib behind it to which it is attached by fibrous ligaments. The spinal ribs are con- nected with the sternum by true osseous sternal ribs, which have regular articulations at each end, so as to allow freer motion to the sternum for the alteration of the capacity of the thorax. The sternum is more remarkably developed in Birds than in any other Vertebrata : being so augmented in length and breadth, as to cover the ventral surface of a great part of the abdominal as well as of the thoracic cavity ; and usuall v having its central part elevated into a keel or ridge, the degree of pronii nence of which bears a regular proportion to ^strength of the pecCal muscles, and consequently to the power of flight. The ribs being devf loped from all the vertebra between the scapular and pelvic arches thl usual 'lumbar' region would «a*™ +„ k„ JLu— ;„ -d- -. T ae b . the region would seem to be wanting in Birds ; but this probably due to its absorption (so to speak) into the sacral' reo-i IS ion; fox 88 GENERAL PLAN the iliac bones are prolonged very far forwards, and are connected with a far larger number of vertebrae than we elsewhere find giving support to sacrum less than 10, and being augmented, especially in Birds whose support and locomotion chiefly depend upon the posterior extremities, even to 19- — The scapular arch is still formed upon the plan which prevails through the lower Vertebrata ; the principal connection of the scapula with the sternum being effected by means of the coracoid bone, whilst an independent clavicular arch, the degree of development of which varies greatly in different tribes of Birds, is formed by the union of the two clavicles on the median line, constituting the ' furcula.' The anterior extremity is chiefly remarkably for the longitudinal extension and lateral consolidation of the bones of the hand ; only two carpals and two meta- carpals being distinguishable (the latter anchylosed together), and only one digit attaining any considerable development (Fig. 2) though rudi- ments of a thumb and of one or two additional digits are usually trace- able. The osseous framework- nf the win attachment of muscles, and for the support of the integument in which the wing-feathers are implanted ; it being by these, and not by any con- tinuous expansion of the skin itself (as in Bats and the extinct Ptero- dactyles) that the required surface is afforded. The pelvic arch is not merely remarkable for the great number of sacral vertebra? which are anchylosed to form its support, but also for the separation of its two lateral ^ halves at the median symphysis ; its incompleteness at that part appearing to have reference to the relatively-larger size of the eggs of Birds, whilst the extraordinary elongation as well as peculiar firmness of the sacro-iliac symphysis would seem intended to compensate for this deficiency. In the conformation of the lower extremity, we have chiefly to notice that the femur remains short, even when the general elonga- tion of the limb is the greatest ; it being in the tibia and metatarsus that the greatest variations occur. The fibula is rarely present as a separate bone, being usually anchylosed with the tibia, and being some- times almost undistinguishable. The tarsal bones are anchylosed with the metatarsal at an early period in the life of most Birds ; and the metatarsus appears to consist of but a single bone, although there are indications that it contains the elements of three, these remaining distinct in the Penguin for a considerable part of their length. The foot usually possesses four digits, with Sometimes the rudiment of a fifth; but in some of the Struthious birds we find the number reduced to three, or even two. The number of phalanges is five in the fourth or outermost di«it but diminishes regularly to two in the first or innermost. 67. As might be expected from the analogy of Birds with Insects, and from the large proportion of their body that is occupied with the appa- ratus of locomotion, the organs of nutrition are comparatively small ; but what is wanting in size is made up in functional activity. The remarkable development of the instinctive propensities is another inte- resting point of correspondence between the two classes; there being this difference, however, between them, that whereas the actions of Insects appear to be entirely governed by these propensities, those of Birds are modified by their intelligence. In respect to this attribute, as > ■ ■ i r<» •• GENERAL VIEW OF ANIMAL KINGDOM. — MAMMALS. 89 ' in the development of the Cerebrum which is its instrument, Birds appear to be strictly intermediate between Reptiles and Mammals; and in the general structure of their sensorial apparatus a like posi- tion is indicated, whilst in particular points of conformation they differ from both these classes. — It is interesting to observe how, without any essential departure from the type of Generative apparatus which prevails among the lower Vertebrata, a much higher physiological charac- ter is here imparted to the function. The store of nutriment provided in the egg for the embryo, is very much larger in proportion to the bulk of the animal ; so that its development can be carried-on to a higher point, before it is thrown upon its own resources. Again, the evolution of the germ, and its appropriation of the nutriment thus provided, are promoted by the high temperature which is constantly imparted by the body of the parent. And even after its emersion from the egg, the young Bird generally remains for some time more or less dependent upon its parent for warmth and nurture ; this dependence being usually most prolonged and complete, in Birds whose faculties ultimately attain the highest elevation. ■68. The Mammal Man oup but also as possessing the most differentiated organization, adapted to perform the greatest number and variety of actions, and to execute these with the greatest intelligence. This high development is obviously con- nected with that greatly-prolonged connection between the parent and the offspring, which is the special characteristic of the class. The ovum Mammals store of nutriment which it contains, serves only for the earliest period of the developmental process ; but in the later stages of this process, the embryo draws for itself a continual supply from the circulating fluid of the parent, by means of a new and special apparatus ; and after this has ceased, and it has come into the world alive, it is nourished for some time longer by the mammary secretion. The degree of development which the foetus has attained at the time of its birth, however, varies consi- derably in the different orders, as does also the completeness of the appa- ratus by which the foetus draws its nutriment from the parent ; and hence is founded the division of the class into the two sub-classes of Placental and Implacental Mammals — the special apparatus for the nutrition of the foetus of the latter never attaining the character of a true ' placenta ' and their whole organization presenting many points of affinity to that of the oviparous Vertebrata. Here, again, therefore, we have a marked illustration of the general conformity between the grade of development ultimately to be attained, and the degree and duration of the parental assistance early afforded to the embryo ; and this is further manifest in the general correspondence which may be noticed, between the decree and duration of the dependence of the young animal upon its parent after birth and the elevation of its subsequent condition. Those which are earliest able to obtain their own food and to keep up an independent temperature, make (for the most part) but little advance beyond that point; whilst the highest development of the cerebrum, and the most decided indications of intelligence, are met with among those whose / 90 GENERA! ORGANIC STRUCTURE AND Man period of self-support is the longest postponed. prolongation of the period of infancy has a most important and Tviclent influence upon the social economy of the race. I I ft fill ~~ T* ; r T «■ a ^ Mammal , taken as a whole, is not characterized so much by the possession of any one particular faculty— like that which has been seen in Birds— as by the perfect combination of the different powers, which renders the animals belonging to it susceptible of a much greater variety of actions, than any others can perform. There are none that can compete with Birds in acuteness of sight ; but there are few that do not possess the senses of smell, taste, and touch in a more ele- vated degree. There are none which can rival Birds in rapidity of loco- motion ; but there are few which cannot perform several kinds of pro- gression. Their inferior energy of muscular movement is accompanied by an inferior amount of respiration; the type of the respiratory appa- ratus, however, is higher than in Birds, a like extent of surface beL comprised within a smaller space. The lungs are confined to the cavity of the thorax ; and there is a provision for the regular renewal of the air received into them, by the action of the diaphragm, which here com- pletely separates that cavity from the abdomen. The diminished amount of respiration, again, involves the production of a lower degree of animal heat; so that the temperature of this class seldom rises above 100° There is less need of means, therefore, for effectually confining their caloric — especially, too, as their greater average size causes their radiating surface to be much less in proportion to their bulk, than is that of Birds* and accordingly, we find them provided only with a covering of hair or fur, which is much less warm than that of feathers, and which is usually thin and scanty in Mammals inhabiting tropical climates. In the Cetacea, which are animals adapted to lead the life of Fishes, the same end is answered by the interposition of an immense quantity of 'oleaginous matter in the meshes of their enormously-thickened skin ; thus forming the < blubber,' which constitutes an admirable badly-conducting septum between the warm body of the animal it incloses and the cold water of the surrounding ocean. - -x &J VA Mammalej as compared with Birds, renders it unnecessary that the nutritive func- tions in general should be carried-on with that extraordinary activity which characterizes the last-named class. Accordingly we find that the demand for food is less constant, the digestive process is less rapidly accomplished, and the circulation is slower, than in Birds ; whilst on the other hand the ' waste' of the body, as indicated not merely by the amount of carbonic acid set free, but by that of the other excreta (espe- cially the urinary), is less considerable, although the organs by which it is eliminated are developed (like the respiratory apparatus) upon a higher 70. Although the skeleton of any ordinary four-footed Mammal pre- sents a strong general resemblance to that of a Lizard, the mode of loco- m otion being the same in both cases, a considerable advance in the «rade of development is observable, when the osseous framework is more closely examined. This is remarkably the case with regard to the inti- mate structure of the bones themselves, which are conformable to the Reptilian type (in the absence of any well-marked central cavity) at an early period of their evolution, but in which a dense shaft with a hollow iMMMMB »JP».V GENERAL VIEW OF ANIMAL KINGDOM. — MAMMALS. 91 interior is afterwards substituted for the cancellated tissue which, at first prevails throughout ; and it is the case also with regard to the consolidation which the skeleton undergoes, by the gradual union of parts originally deve- loped from distinct centres. The skeletons of the non-placental Mammals, however, present many interesting links of affinity to those of Reptiles ; as do also those of the gigantic extinct Sloths, between which and the Dinosauria there seems to have been a mutual approximation. — The con- formity of the different orders of Placental Mammals to one plan, as shown in the structure of their skeleton, is much greater than it is in Reptiles. We never find the extremities entirely suppressed, as in Ser- pents ; nor are the ribs anywhere absent, as in Frogs ; nor is there any such junction of the dermo-skeleton with the neuro-skeleton, as presents itself in the Turtles, although the former may constitute (as in the Armadillo) a complete bony envelope. Still, the modifications which do present themselves, are far more considerable than are exhibited by the class of Birds ; and these have reference, as in Reptiles, to the adaptation of Mammals for residence in the water, like Fishes, or for passing a large part of their lives on the wing, like Birds. In the Cetacea, the power of locomotion is almost entirely taken away from the extremities, and given back to the trunk, as in Fishes, the posterior extremities being entirely deficient, whilst the anterior serve only for guidance ; there is this important difference, however, that the tail, which is flattened vertically in Fishes, is flattened horizontally in Cetacea, which, being air-breath- ing animals, need the power of frequently coming to the surface to respire. In the Cheiroptera, on the other hand, the power of locomotion is almost entirely delegated to the anterior extremities, most of the bones of which are of large proportional dimensions, the principal elongation, how- ever, being in the bones of the hand (Fig. 2, e) ; and their skeletons exhibit various other adaptive modifications for the same end, the sternum, for example, having a prominent keel, and the pelvis having its arch incom- plete, as in Birds. But although the paddles of the Whale may remind us, in position and external aspect, of the pectoral fins of a Fish, — the rudimentary condition of the neck causing them to be situated just behind the head, and the inclusion of the short arm and fore -arm within the trunk leaving only the bones of the hand to support the undivided skin, — the essentially Mammalian type may be traced in all the parts of the skeleton of these members (Fig. 2, c) ; the only real approximation to a lower grade consisting in the multiplication of the phalangeal bones, as in the Enaliosauria, the digits themselves never exceeding their regular number five. The same constancy to the Mammalian type shows itself in the bones of the anterior extremity of the Bat; which though modified for exactly the same purpose as that of the Bird (so that bats and swallows replace one another in the pursuit of the same insect-prey') attains that purpose by a different plan of organisation. i 7 \ ^ he /S 0wing ' are the most im Portant distinctive peculiarities in the skull of Mammalia. The cranial cavity, which is of a rounded form is shut-in by a set of bones, which, while formed by the partial consoli- dation of the elements of the neural arches of the cranial vertebra remain separate during the whole of life, distinct sutures being interposed between them ; the bones of the face are usually but little developed m proportion, save m certain tribes in which the skull is much elongated mm ^^_h_^H iHHHB 92 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT in consequence of the great development of the jaws, and they are all (with the exception of the lower jaw) immovably articulated to each other and to the cranium; the lower jaw articulates directly with the cranium, instead of by an intermediate moveable tympanic bone as in oviparous Vertebrata, this piece having coalesced with other vertebral elements to form the i temporal bone ;' and the articulation of the cranium with the spinal column is no longer effected by the centrum of the occipital vertebra, but is formed by two lateral surfaces, corresponding with the articulating processes of ordinary vertebrae. — The number of cervical vertebrae is almost invariably 7; and this is not altered, either in the Cetacea, which have a very short and immovable neck, or in the Giraffe, whose neck is enormously elongated. The number of dorsal or rib-bearing vertebrae usually varies between 11 and 20; the ribs are always articulated, save in the most Reptilian Mammals, both by their ' heads' with the bodies of the vertebrae, and by their ' tubercles' with the transverse processes ; the costal cartilages or sternal ribs usually remain cartilaginous, although they sometimes become ossified in advancing life; and they are for the most part undeveloped in Cetacea, the freedom of whose spinal ribs (with the exception of the first pair or two) reminds us of Fishes. The development of the sternum varies very considerably in different groups, chiefly in accordance with the demand for muscular power in the anterior extremity ; thus it is not only in the Mole that it possesses a central projecting keel, but also in Moles and Armadilloes, whose fore-feet are used as spades in burrowing. It is not ordinarily prolonged over the abdomen, the abdominal sternum and ribs of Reptiles being represented only by longitudinal and transverse aponeurotic bands ; but this portion is unusually developed in the Edentata. The ordinary number of lumbar vertebrae is either four or five; that of the sacral varies more considerably, the number of vertebral segments consolidated for the support of the pelvic arch, generally showing a proportion to the degree of strength which the members it bears are formed to exert. Thus in the Cetacea, which have no posterior extremities, the union of vertebrae into a sacrum is altogether wanting, as in Fishes ; in Man and most of the higher Mammalia, on the other hand, five or even six vertebrae are consolidated : yet in the implacental Mammals, the Reptilian number two uniformly presents itself, notwithstanding that in many of them, as the Kangaroos and Opossums, the posterior extremities are enormously developed, and are the principal instruments in progres- sion. The number and development of the caudal vertebrae vary more widely than do those of the segments of any other region ; for whilst in Man and the higher Apes there are no more than i or 5, these being abortive and becoming anchylosed with each other so as to form the ' coccygis,' the number rises (in other Mammalia) to 20, 30, or even 40. It is interesting to remark that the highest numbers occur in those orders (the Marsupialia and the Edentata) which present the greatest number of other approximations to the Reptilian type of structure ; and in some of the former of these, we find a very curious reversion to the Reptilian type, in the presence of haemal arches attached to the bodies of the caudal vertebrae. As a general fact, however, the tail is a part of the vertebral column, whose development or non-development does not follow any general plan, but is related to the special use to be made of it in the OS aHHHH l GENERAL VIEW OF ANIMAL KINGDOM. — MAMMALS. 93 economy of the animal; and of such uses the Mammalian class presents a remarkable variety. The conformation of the scapular arch in Mam- mals generally differs from that of Oviparous Vertebrata in the inferior development of the coracoid element, which is not of sufficient length to reach the sternum or to meet its fellow on the median line ; the osseous connection of the scapula with the sternum, where such exists, being established by the clavicle. In the Monotremata, however, with other reptilian affinities, we find this connection to be established both by coracoid and clavicle in a most peculiar manner, through the intermedi- ation of an c episternal' bone ; the whole clavicular arch having a most remarkable resemblance to that of Ichthyosaurus. The three com- ponent bones of the pelvic arch generally coalesce together at an early period of life, remaining separate, however, in Monotremata ; the inferior symphysis is complete (save in Bats), being entirely formed by the junction of the rami of the pubis; but in the Implacentalia, as in Reptiles, the ischia have a share in the junction. The extremities attached to these arches exhibit a great variety of special modifications of structure ; putting aside, however, the extreme modifications presented in Bats and Whales, we recognize two principal sub-types, the ungulated and the unguiculated. In the former of these (Fig. 2, d), the members, being adapted simply for support and locomotion, are developed in a form which renders them most efficient instruments for these purposes. All the articulations, even those of the shoulder and hip-joints, are so constructed as to limit the movements of the limb to that one plane (back- wards and forwards) in which its actions are required for the onward propulsion of the body ; the bones of the fore-arm and leg are consoli- dated into one, so as entirely to prevent any rotation of the hand or foot ; and only a pair of digits, or even a single one, are developed in each member, these being entirely enveloped in hard horny casings. The opposite extreme is where, as in Man (Fig. 2, f), the form of the humeral articulation, and the arrangement of its muscles, confer upon the anterior extremity (the posterior partaking of the same endowments, but in an inferior degree), the power of free and extensive motion; the plane of the hand can be turned in any direction by the rota- tion of the fore-arm and the flexure of the wrist; the whole number of digits is developed, and one of them is so opposed to the rest as to be capable of antagonism to either one or to all of them collectively; and the extremities of the fingers are covered by the nail on one side only, leaving the other possessed of the highest tactile delicacy. Between these two extremes, there is an immense variety of intermediate gradations. 72. The teeth of Mammalia constitute a remarkably characteristic feature in their organization ; and the differentiation which they exhihit in the several orders and genera is so great, and is so closely connected with other peculiarities, as to afford most important assistance in classifi- cation. They are, for the most part, much less multiplied than in Ren- tiles ; and when the typical number 44 is exceeded, it is in those groups which either represent Fishes, or make the closest approximation to Bep- tiles ^ and it is m these, moreover, that the teeth, having the least deo T ee of individual development, present so little differentiation, that they not be classed, as they may be in all the higher forms of the dental can- 94 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. apparatus, into incisors, canines, premolars, and molars. — In the mode of implantation of the teeth of Mammalia, we have a marked distinctive character of the class ; for in all save those which grow from persistent pulps, we find the dental cavity closed-in at its lower part, and the base of the tooth prolonged into a < fang,' which is implanted into its own proper socket, but not united by ossification to its bony wall. The fang of molar teeth is usually subdivided into two, three, or even four port tions, which diverge more or less from each other, and are received into separate divisions of the alveolar cavity ; and this mode of implantation is so peculiar to Mammals (as far as at present known), that its existence appears sufficient to determine the mammalian nature of a j aw, or even of a fragment of a j aw, in which it occurs, such as that of PJmscolotherium Fig. 71. <^ML Lower jaw of Phascolotherium Bucklandii. (Fig. 71), or Amphitherium, of the Stonesfield slate, whose reptilian nature has been advocated by many zoologists.— The teeth of Fig. 72. Mammalia ordinarily cast and renewed but once during life, instead of being continually shed as they are in Reptiles and Fishes, and replaced by new teeth developed from in- dependent papillae, or from off- sets from the previous follicles. This general rule, however, is subject to exceptions in parti- cular cases. For in the Ele- phant, we find the sides of the jaws to be occupied, not by rows of molar teeth, but by a single large composite tooth on either side of each jaw (Fig. 72), this being formed of a succession of alternating plates of enamel, ce- mentum and dentine. The chief wear of these teeth is in front; and a production of new teeth is continually taking place behind, so that each tooth, as it wears down, is pushed forward by the new tooth behind it, which comes to occupy its place ; and the molars are thus changed six or eight times. The ' tusks ' Molar tooth of Asiatic Elephant. I PROGRESS FKOM GENERAL TO SPECIAL IN DEVELOPMENT. 95 on the other hand, though only renewed once, like the teeth of Mammals generally, are in a state of constant growth from the persistent pulps at their base ; and this plan is adopted also in several other cases, in which the teeth are particularly liable to be worn-down by the friction to which they are exposed. 73. Having thus found, in the general survey we have taken of the Vegetable and Animal kingdoms, that the ' idea' of their combined unity and diversity of organization is that of progress from the more general to the more special, we shall enquire how far that idea is conformable to the actual history of their Development. This, when carefully scrutinized, is found to afford the most satisfactory proof of it. — The perfect organism of any one among the higher Plants and Animals is not more dissimilar in form and con dition to that of the lowest and simplest member of either own its evolution ; and in fact, when we go back to the very commencement of the process, we observe that the most general type of organised struc- every ture, — the simple cell, The evolution of the germ commences in the duplicative multiplication of this cell, precisely after the fashion of the multiplication of the simplest Protophyta and Protozoa (Fig. 73). It is not until this has v v X i Multiplication of cells of CHamydomonas (Ehrenb. ) by duplicative subdivision. Fig. 74. A B Fig. 73. proceeded to a considerable ex- I tent, that it could be stated with certainty, from the appearance of the germ alone, whether it is that of a Plant or of an Ani- mal ; thus in the accompanying figure (Fig. 74), the ' mulberry- mass ' of the mammalian ovum, formed by the repetition of the duplicative subdivision of the germ-cell, is shown to bear a most exact correspondence to the Volvox globator (an organism now certainly known to belong to the vegetable kingdom) in its early stage. At the time, again, when the distinctive characters of animality first present them- selves, it could not be predi- cated whether the germ is that of a Radiated, Molluscous, Articulated or Yertebrated animal; the special characters of these sub-kingdoms not being evolved until a later period. These, however, are the next to the a, early stage of Mammalian Ovum ; Volvox Globator. -B, young of appear, but still the distinctive peculiarities of the class are wantim UJllo germ of a Yertebrated animal (for example) being at first destitute of ' wf 1 ^ at :. Ca ?. ma ? {t 011 t as a Fish, Reptile, Bird, or Mammal. When the distinctive characters of the class have been made manifest bv ; the further progress of development, those of the order still remain inde terminate ; these are evolved in their turn ; and then those of the family I 96 GENEBAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. ing what amount of real truth there is in it. genus, species, sex, and individual, in succession.* This, at least, is the result of observations made in a considerable number of cases ; and where such an accordance does not exist, the want of it is probably due to im- perfections in the system of classification with which the comparison is made. — Thus we see that in watching the history of the development of any one of the higher forms of organised structure, we find the realization of that ideal evolution of the more special characters from the more general, which it is the object of the Philosophic Naturalist to bring into view by the methods of proceeding already pointed out (§ 11). 74. The general principle of Yon Baer affords the real explanation of those resemblances which are sometimes discernible, between the transitory forms exhibited by the embryoes of higher beings, and the permanent conditions of the lower. When these resemblances were first observed in the study of Embryology, an attempt was made to generalize them in the statement " that the higher animals, in the progress of their development, pass through a series of forms corresponding with those that remain perma- nent in the lower parts of the animal scale." But this statement was hasty and unphilosophical ; and it is now only referred-to, for the sake of show- [— No animal as a whole passes through any such series of changes, except where it comes forth from the egg in an early stage of development, but in a condition that enables it to sustain its own existence, and to lead the life of a class below, from which it is afterwards raised by metamorphosis. This is the case, for example, with such Insects as resemble Annelida in their larva condition, and with Batrachian .Reptiles, which are essentially Fish during the early period of their lives. But in neither of these instances, does the Larva entirely resemble the perfect animal which it represents in form and grade of organisation ; for besides having its generative system undeveloped (with- out which it cannot be said to be a complete animal), the condition of its tissues and organs is altogether embryonic ; so that the caterpillar bears a much closer accordance with the embryonic than with the adult Annelide, while the Tadpole is more nearly related to the embryonic than to the perfected Fish. These and other cases of the same kind must be regarded as special modifications of the general plan to meet a particular purpose ; and while they present nothing discordant with that plan, they cannot be taken as examples of the usual mode in which it is followed out. On studying the development of any one of the higher animals, which remains within the ovum until it has attained the form character- istic of its class, we find that its entire structure does not present at any time such a resemblance to either of the classes beneath, as would * Although, this general truth had been previously indicated by Von Baer, yet the first definite and complete statement of it, with its application to Classification, will be found (the Author believes) in two papers ' On Unity of Structure in the Animal Kingdom' con- tributed by Dr. Martin Barry to the " Edinburgh Philosophical Journal" for 1837. It has been subsequently developed in a very admirable manner by Prof. Milne-Edwards in a Memoir * On the Principles of the Natural Classification of Animals, ' in the ' ' Annales des Sciences Naturelles," Ser. in., torn, i., which bears evidence of having been written with- out the knowledge of what either Von Baer or Dr. Barry had put forth; the principle, in fact, having been advanced in a more limited form by Prof. Milne-Edwards himself, in a memoir < On the Changes of Form exhibited by various Crustacea during their Develop- ment,' read by him to the French Academy in 1833, and published in the " Annales des Sciences Naturelles," Ser. i., torn, xxx., Ser. n., torn. iii. PROGRESS FROM GENERAL TO SPECIAL IN DEVELOPMENT. 97 justify the slightest analogy; Human „ — ^ v ^ oiigxiucst auaiugy j unus, me .tiuman emoryo is never com- P9 M We With a Fisil ' a Re P tile ' or a Bird > mucn less with aI1 Insect or with a Mollusk. In its very earliest grade, indeed, it might be likened to the + 6 + a °L? luHters of cells > of wnicli ^e Protophyta and Protozoa are consti- tuted (Fig. 73, 74) ; but so soon as the multiplication and conversion of tiiese has proceeded to such an extent, as to give it a form and structure m wnich a resemblance can be traced to any higher animal, it is to the . __ --- ~>>r~ -— ~ "- ««vw*v* c»,u uuw assign it. Now, whilst it is passing through this condition, a close correspondence may be traced be- tween the several parts of its structure and those of any oilier vertebrated embryo at a similar grade of development :— there is, for example, no essential difference between the vertebral column of the early embryo of Man, and that of an embryo Fish ; bhe evolution of the nervous centres oegms m both upon the same plan ; so also does that of the circulating apparatus. And as the progress of development is arrested in the lower tribes, at the stages thus indicated in the transitional conditions of the Higher, a mutual resemblance in the condition of particular organs mav most assuredly hence arise. Thus, in the Cyclostome and higher Cartila gmous Fishes, we find permanently represented the various stages in the development of the vertebral column, which may be detected in the em- co^l^ ^ Vert 1 ebl J ta ; But eacb of *«* animals presents, in its adult ana Sis ' P otl a t P l atl ° n ° f the g6neral P kn t0 its own °^™™-> TW , ViTS, modlficatlon ™ n °t represented in the human embryo numW otV V TT ° f ^Human embryo is developed from a great maneJlv T* "5*^ T^ repreSent tbe Ws tbat remain pei- Sc/t y + r P A m ^ Skul ° f the Fish '- S0 that tbere is a oone^on- t^Z^rf?*^^ ^de^Pment, as presented in the two kSLTK X 7 i'~7Pt \* never exMbits those P ec » liar characters, w' ' " to talTn i W k f ^ FiS \ fr ° m that ° f a " 0ther Vertebrata. Ur, a* an eart t ? i™^ ^^ S0UrCe ' tbe Elation is carried on svstl ^fbLT m i r + de 7 el ° P r nt ° f a11 ^ebrated animals, by a S^t^n^'TSi 8 ^t nb T ted Up ° n the same P lan as that which is tTe cTrcnWi^ 6 , / 1Sh ^ £ ™ ^ Wver ' C ° rrect to affim b that that ofT S g 2®"$™ °[ ^ Ian ever P asse * trough the condition of VerteLat!^;- 1 ^f.^ '. brancbial ar ^es' are developed in all ZftlettnJ f f S ' 1° ^ tLeir prGSence m ^ be considered as the Xcb^Z 2? • the 1 hurtOT y ° f their arterial s 7 stem > 7^ the twigs wh ch they give off m the adult Fish for the supply of the branchial Sr a^r neVer + . devel °P ed > except in animals lUt are to be Xte] tnro2h tL T Pir v°f > S t ^ the bl °° d flows onwards continuous^ tnrougn the branchial arches, and is delivered bv tliPm ,•«+,* +1, , J instead of being distributed amongst aTSSTtob^i^ ^ T^ ^J?^^ «*_ .ff T-* ^ Whiaf veins° 'w !l !! Or, Man ffafrf Bid j sice eZy TZt^T !? m ^° rf *"» and the »- which gives to the «S^^T' t ^ ^, g !5^. ?^™ ' ^V*'' does in reality diminish the resenihLT w. ch f Mt ™ s t'<; s ° f * class, either of the higher classes. ThTwhuTth,, * J ^ eml "7 of close correspondence between tS^M 1 ^ ^ T™? a Ter y hannonv with the genera, principlet^n S, tZ.nl I^t ? II I : 98 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. proceeds in its course of development, takes a direction that separates it from the rest ; and the mutual divergence consequently becomes greater and greater, in proportion as the perfected form and condition, that are characteristic of each class respectively, are approximated. 75. Now although the life of all Organised beings commences in the simplest and most general type of organic structure, so that there is no perceptible distinction between their germs, yet we see that each germ must have a certain capacity of development peculiar to itself ; since it is a general law of Organic Development, that like produces like. However varied may be the series of forms through which the parent passes, the offspring repeats these with the greatest exactness f and the whole scheme of development may be described as one in which the primordial cell is tending towards the attainment of the perfect form and condition of its parent. In proportion to the mutual resemblance of the parents, will be the conformity of the processes by which their respective forms are at- tained ; in proportion to the dissimilarity of their adult conditions, will be the divergence of their directions of development : thus the develop- ment of the heart of the Bird and of the Mammal proceeds upon a method essentially the same, the single ventricle being divided first, and the single auricle subsequently, the septum remaining imperfect in the Mammal until birth ; but in the Reptile the auricle is first divided, its circulation being carried-on upon a plan to which the embryo Bird and Mammal never present anything comparable. And in accordance with the degree of proximity of each complete form to the general model or ' archetype ' of the entire series, will be the degree in which it will be represented in the transitional states of the higher forms: thus the vertebral skeleton of the Fish as a whole departs much less from the h and consequently, that of the Mammal is to the latter. — These examples will serve, it is hoped, to show the dis- tinction between the fundamental principle of development, first enun- ciated by Yon Baer, and which is applicable (as the author believes) to all the facts hitherto ascertained, and that crude and illogical generalization which has brought discredit upon Philosophical Biology, and has led to a host of erroneous inferences, t * It will be shown hereafter (chap, xi.) that the phenomena ranked under the term 4 Alternation of Generations' do not constitute a real exception to this rule. + It is owing to the ignorance of Von Baer s writings which has generally prevailed in this country, that the credit has been recently assigned to others, of having first enunciated the true view of this subject. The Author may refer to the second edition of the present work, published in 1841, as having contained the doctrine stated above, which he was also accustomed to teach in his Physiological Lectures ; and although his own acquaintance with Von Baer s works at that time extended but little beyond the references made to them by Dr. Martin Barry, yet these were sufficient to enable him to comprehend and apply the great developmental law which Von Baer had so clearly enunciated, and to lead him to the very same illustrations as those which he afterwards found that Von Baer had employed. He cannot but think that the admirers of the great English comparative anatomist of our own time, would have done well to abstain from preferring on his behalf any claim to origi- nality on this subject, until they had ascertained how far he had been anticipated by others. In the " Quarterly Review" for October, 1851 (vol. lxxxix. p. 430), Prof. Owen is spoken of as having ' ' first distinctly enunciated the generalization, that in the development of the vertebrate animal, the germ passes at once from the common form of the protozoon or monad to the vertebrated type, without transitorily representing either the radiate, articu- late, or molluscous types." Now in Von Baer's great work ' ' Uber Entwickelungs-Greschichte PROGRESS FROM GENERAL TO SPECIAL IN DEVELOPMENT. 99 tll J?* /* ls maintained, indeed, by some distinguished Naturalists, that ill ™niation derivable from the history of Development, in regard to I™ 6 Vah f oi . ^aracters and the affinity of groups, is so much ST a ? d f tisfact °ry than that of any other kind, that it ought •S* the fun , dam ^ ta l d^a for a truly scientific classification : those Wnlv^ ?° nSlde f d 7 as most * ea % related to each other, whose em- fc?!Sr{ advances furthest along the same course without tXr + T ^ ^ bG regarded as most fimdamentaUy dissimi- lar, whose directions of development are distinct from the earliest period ±his principle may be admitted as one which deserves to be fullv taken into account m any attempt at a systematic arrangement on philoso- phical principles j but to adopt it to the exclusion of all comparison of forms m their state of complete evolution, would be to deprive fmportant changes which may occur at a comparatively late period of deveTopment of their due c aim to consideration. The following illustrative ex- ^IZTL^I t0 m ^ its *™ I al - apparent.^ the class" Milne such as come forth from the e gg ^^^fy^^S'S^^Z have many changes to undergo, resemble one another very cC As whTciX aSG m I"?' W T' the P^ lia -tie S of the respective tribes to which they may belong, gradually manifest themselves, partly throul an etlution rf ™ "Y **!**»«<* of different parts "and part ty by th evolution of new and special organs. Thus in one case, it is the thorax dimensions in ot W 7^ WhlCh ^^ the « reatest in ° rease in ** seeT?n Si^ Pvt .^tances, again, an extraordinary development is tremities So ^ remi * ies ' °* eve » m certa ^ articulations of these ex- possessed hi % g < V"* Uncommon for certai » P a ^ which were P^to^JSr T^° ? rU j tacean > t0 become ^ophied and to disap- Se,L 7 ther J endmg t0 ****»&* the particular form in its theC^^ J rf* 6 de T el °P ment - X » «*» and other modes, ser^S ' i ' he tlme 0f lts emer sion from the egg, may have pre ti; otr s T- s rv° ******* {t frS ^ ia -- of *^S tWl 7 . dlssl f dar fif 1 ^ gradually comes to present in TIT^T Y C }^ Gt f. s of lts **», genus, species, and sex.* " £ O^T P 6 °V^ S l md ° f S P ecializatio » * afforded by certain para- a W^T + f a the Entomos tracous group ; which have their forms so ^erea by the enormous development of their ovaries (Fig. 75 b a a\ as well as by the evolution of egg-sacs (b, b), and by the modification of their appendages for adhesion and of their mouth for suction, that thev were long ranked in a distinct group, under the designation of E^ol their ongmal type being almost obliterated. Yet it if now known that ^^^^^M^^S^^ very doctrine occupy the 4th section embryonic mass, with the pamJK^M^ Je rdatum of the earliest states of the that the chorda dorsalis, the SdWbSSi? £*♦ 7f,' ¥ ^ been detected i but first part that is difcr4tiat^lS*ttS^W ^ Vertebrate em ^°- * the correspondence to the special radkte artiJ„f Jf "i^^ f™* b ? arB the sli § htest again most emphatically asserted artlculate > or molluscous types, 1S over and over ^ m tne Cyclopaedia of Anatomy and PWsinW,r » _i • A very Cyclopaedia of Anatomy and Physiology H 2 vol, i. 100 PLAN this modification is exhibited by the females alone ; the males (a), which are often so small as to be mistaken for parasites upon the female, con- tinning to exhibit the ordinary Entomostracous type, that of Nicothoe Fig. 75. E A, male of Nicothoe astaci : — b, adult female of the same species, having two large lateral appendages, a, a, containing the ovaries, as well as two egg-sacs, b, b ; — c, young larva viewed sideways ; — D, more advanced larva, provided with all its members ; — e, larva already fixed, the lateral appendages beginning to appear ; — r, further development of the same. bearing a close resemblance to Cyclops (Fig. 60) \ and the change in the female only taking place gradually, as her generative apparatus evolves itself (c, d, e, f).* — If we turn, on the other hand, to the Cirrhipeds, we find that whilst they closely agree with Crustacea in their larval condi- tion, and must, if the principle of development be followed as the sole guide, be placed at no great distance from the Entomostracous sub- class, if not as actual members of it, they undergo such extraordinary metamorphoses at a later stage, that their character is changed in a de- gree sufficient to exclude them from any definition, however comprehen- sive, which may be framed for that class ; whilst they are brought into much closer approximation with the Mollusca, than is exhibited by any other group of the Articulated series (§ 14). * Van Beneden 'Surle D e veloppement et 1' Organization des Nicothoes,' in "Ann. des Sci. Nat.' 3iemeSerie, Zool., torn, xiii., p. 354, et seq. MEANING OF KUDIMENTARY ORGANS. 101 fr I A J hUS ' ^^ wli ether we compare the whole assemblage of per- ectecl iorms which make-up any one group, or examine into the progres- sive changes which they respectively undergo in attaining their complete development we find that their differences essentially consist in the 1 dative n P.vpI nrmi on + r^f +1^ ^ ^1 ~~ x i •■_ i . 11 . J Hence - -p^n -p n x r .,; ~~^" wo yvxxxv,n an pu»btj»s m common. ±ience. S^SS^Sr 2* M rrf t rence . *°. the tt-ta-,* « arrive at different habits of rfany iriJeJ -f™ „ *i ^7 ;" „ — ^ T^^ wi '"wnur aroups are pro- bvt f Z'Z?f- the . er T° n 0fneW ° rganS ° r the Auction of others, but la to ZfT r!? /0 T' StrU ? Ure .> W V U <*> °f <"9™ typically belong- Z 9 J A f T P -~ ThuS the P roboscis of tte Elephant, which constitutes so wonderful an instrument of prehension, is but an extended nose and extension mg Mammals as well as 0nd & ^ & w _ „.^on ox tne cranium) by ^various extinct Pachydermata. The wing of the Bat, as we have W is not an additional member, but is stretched upon an extended haud' that of the Pterodactylus, upon a single finger. The neck of tW ,S' contains no additional verteLe; bufis adfpted tXlm^t^Z^ modifications m the structure of the limited 'number tvoical of +1^2 arelSLre pt Ed S^ "^ ^Tt M th ° se & °» ^cn teetn tW ZT P r <»nced.— fco, turning to the Vegetable kingdom w e find tions to the ordinary faW ' * n0t mtroduced as addi- that 8 'if A fCXn fin o d / « ^f ^ ^ been 6Xpected Under the fore g°^g ^w, develop nf stract ^ in a particular tribe involves the non- ^^S^ ?-. - - .' ladder' in regnlating^ abdominal sternum and ribs of Saurian Mammals Mammal tt&toiirz te-^H^^S &."* arcn which is elsewhere completely ossified. tures, however, often display themselves uuxy at an development, and are subsequently lost sight of rudimentary struc Thus the rudiments of teeth, which are never developed and E; t x ttle . rudlm ents of detected, are found in thllS^ Ift^t ?* ? ^ P eriod ca ^ot be found in the embryo of the Whal under jaws; and Prof Good™ ■>,««, 7 I • '-, , m the u PP er an d canine 'teeth and of the mcS s fe?^ ^V^ ^^ of quently developed, exist TIL °^. Upper J™> which are ** «ubse- t Report of the British Association," 1839, p. 82. Mammals, f t | v- : 102 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT — The most remarkable example of this kind, however, is the existence of branchial arches, resembling those of the Fish, in the early embryo of all air-breathing Mammalia, as will be hereafter explained (chap. vl). — The same is true, as a general rule, in the Vegetable kingdom ; thus when a whorl or part of a whorl in a flower is suppressed, the deficiency is manifested, either by the presence of the undeveloped organs in a rudi- mentary form, or by the leaving of a space for them (so to speak) in the arrangement of the parts which are present. Thus in the Primrose tribe, we commonly find a single row of stamens opposite to the petals, instead of alternating with them, according to the regular plan of floral develop- ment (§ 30) ; and hence the Botanist would conclude that a whorl has been here suppressed, which ought to intervene between the petals and the stamens. This is found to be the case in the genus Samolus, whose flower, formed in other respects upon the same type with the Primrose, possesses the rudiments of the intermediate row, in the form of a whorl of little scales, not developed into stamens. In the common Sage, again, we find only two stamens, where the general plan of the flower would lead us to expect five ; but upon looking attentively at the interior of the corolla, two little scales are often to be seen growing in the place where two of the deficient stamens should have been ; these two scales are frequently developed as perfect stamens, in flowers which are other- wise constructed precisely like the Sage ; and even the fifth makes its appearance in some instances, exactly where it should be regularly found. Sometimes, again, the conformity to a common type is manifested by the full development, under cultivation, of organs which are not usually evolved : thus there are plants in which one set of flowers is purely ' staminiferous,' from the non-development of the carpellary whorl, whilst another set is ' pistilline' only, from the non-development of the stamens ; and in which the effect of increased nutriment is to develope the deficient carpels in one set, and the deficient stamens in the other, so as to render both of them complete and i hermaphrodite.' 79. The attempt has been made to bring the diversities in the propor- tional development of organs which different animals possess in common, under a general expression, — the balancing of organs ; else than that which is alluded-to by Paley and other authors, as the which is nothing objectionable form: principle of compensation." This has been stated in the following most ~ that the extraordinary development of one organ occasions a corresponding deficiency in another, and vice ^ versd. It is perfectly true that, in a great maj ority of cases, the extraordinary develop- ment of one organ is accompanied by a corresponding deficiency of develop- ment in another; but the development and the deficiency are both of them parts of one general plan, and neither can be regarded as the cause, or as the effect, of the other. Thus, in the Human Cranium, the elements which form the covering or protection of the brain are very largely deve- loped, whilst those which constitute the face are comparatively small. In %yJ ^ the long-snouted Herbivorous Mammals, as in Reptiles and Fishes, on the nV x. other hand, the great development of the bones of the face is coincident v -with a very small capacity of the cerebral cavity. In the Bat, whilst the v anterior extremity is widely extended, so as to afford to the animal the V %g means of rising in the air, the posterior is very much lightened, so as not ** to impede its flight. In the Kangaroo, on the other hand, the posterior ■ BALANCING OF ORGANS. HARMONY OF FORMS. 103 members are very large and powerful, enabling the animal to take long leaps; whilst the fore paws are proportionally small. ™ Mole requires for its underground burrows the power of excavating with its fore-feet, whilst the hind legs are used for propulsion only; and the rela- tive development of these members follows the same proportion as in the Bat, although the plan in the two cases is widely different. Moreover it is obvious that, from the peculiar habits of this animal, eyes would be of little or no use to it ; and accordingly we find them merely rudimentary, and no cavity in the skull for their reception ; whilst to compensate for the want of them, the organ of smell, and its capsule — the ethmoid bone, are amazingly developed. In other classes of animals, similar illustra- tions abound ; thus, the Birds of most active and energetic flight usually So, Struthious .__ 7 the legs are enormously developed, have only rudimentary wino-g. again, among Reptiles, we find the vertebral column most lengthened, „*«* the tail especially developed, in those whose limbs are feeblest, or altogether deficient, as among Serpents, and Serpent-like Sauria and Batrachia ; whilst, if the limbs are the principal instruments of locomo- tion, as in Frogs and Turtles, the vertebral column is shortened, and the tail contracted. And in Fishes, the same general rule holds good.— In no class, however, is this rule without its exceptions ; and it must be taken rather as an expression of facts, possessing a certain empirical value, than as entitled to the character of a ' law' of development, which some would claim for it. 80. Another principle, propounded by Cuvier, and supported by those who have adopted the < functional' or < teleologies!' (purposive) rather than the < homologies!' relations of organs as their guide, is that of the if forms, or the coexistence of It implies that there is ■ a. _ a necessity, arising out of the conditions of organic existence, for the combination of organs according to their several actions ; that there is a constant harmony between organs which are functionally connected j and that the altered form of one is invariably attended with a corresponding alteration m the others. A general comparison of the skeleton of a ^arnivorous with that of an Herbivorous quadruped, will furnish a charac- teristic illustration of this doctrine. furnished with a cranial cavity of considerable dimensions, in order that the size of the brain may correspond with the degree of intellect which the habits of the animal require. The face is short, so that the power of the muscles which move the head may be advantageously applied. The canine teeth are large and pointed ; whilst the molars have sharp edges, adapted only for cutting, to which purpose they are most effectively applied by the scissors-like action of the jaw. The lower jaw is short, and the cavity in which its condyle works is deep and narrow, allowing no motion but that of opening and shutting; the fossa in which the temporal muscle is embedded, is very large; and the muscle itself is attached to the iaw in such a manner as to apply the power most advantageouslv to the resist- ance. The spinous processes of the vertebra of the back and neck are very strong and prominent, giving attachment to powerful muscles for raising the head, so as to enable the animal to carry off his prey The bones of the extremities are disposed in such a manner, as to allow the union of strength with freedom of motion ; the head of the humerus 104 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. I round, and the articular surfaces of the fore-arm indicate that it possesses the power of pronation and supination. The toes are separate, and armed with claws, which are retracted when not in use by a special apparatus that leaves its mark upon the hones.— On the other hand, in the con- formation of the Herbivorous quadruped, we are at first struck by the diminished capacity of the cranium, and the increased size of the bones of the face. The jaws are long, and the lower jaw has a great degree of lateral motion, the glenoid cavity being broad and shallow ; and whilst the ptery- goid fossa, m which the muscles that rotate it are lodged, is of large size, the temporal fossa is comparatively small, no powerful biting motions being required by the nature of the food or the mode of obtaining it. The front teeth are fewer and smaller ; but the surfaces of the grinding teeth are extended, and are kept constantly rough by the alternation of dentine and enamel. The limbs are more solidly formed, and have but little freedom of motion, the hip and shoulder being scarcely more than hino-e- j oints ; the _ extremities are encased in hoofs, which are double if the animal ruminates, and either single or multiple if it does not. The whole body is heavier in proportion, the nutritive system being more compli- cated; and the muscles which enable the tiger to lift considerable weights in his mouth, are here necessary to support the weight of the head itself. 8 1 . That this statement is true so far as it goes, no one can deny • and the researches which have been based upon it have been most successful in repeoplmg the globe, as it were, with the forms of animals which have long been extinct, but which can be certainly predicated even from minute fragments of them. A little consideration, however, will show that the existence of such adaptations of parts is nothing more than a result of the general plan of development, and gives us no information of the nature of that plan. It is evident that, if it were deficient, the race must speedily become extinct, the conditions of its existence being no longer fulfilled ; and that whatever be the laws of development, they must operate to this end, in order that the world may be peopled with life. An animal with the carnivorous propensity of the Tiger, for instance, and the teeth or hoofs of a Horse, could not remain alive from the want of power to obtain and prepare its aliment ; nor would a horse be the better for the long canine teeth of the tiger, which would prevent the ;'n •-•"V • i j / Skeleton of Palceotherium magnum. but more with the Horse, of the present epoch) had the complete typical dentition, with three well-developed toes on each foot; but a later species approached the horse more closely, in the reduction of the outer and inner toes, leaving the central one much larger in proportion ; and in a still later species, the outer and inner toes are much more reduced, and the form and proportions of the rest of the skeleton and teeth are brought much nearer those of the Horse, which, in the full development of only a single digit of each member, as well as in the suppression of some of the teeth 'and the remarkable development of others, must be considered as one of the most highly specialized forms of the order. So H l ^— . GEOLOGICAL SUCCESSION OF ORGANIC LIFE. Ill - Mammal day, the most special characters of the group are wanting, or are very teebly manifested. Thus the Bicobune, Dichodon, and Anoplotherium, winch may be inferred from the structure of their teeth and from the conformation of their feet to have been ruminating Mammals, were unpossessed of horns, had canines and incisors in both the upper and lower jaws, and retained through life that separation of the two metacarpal and metatarsal bones, which exists in the true Euminants only during the embryo state, these bones subsequently coalescing in them into the single < cannon-bone.' Hence, as Prof. Owen has remarked, they depart less widely from the archetype, than do the existing Ruminants ■ and are more nearly allied to the embryo-states of the latter, than to their adult forms. Again, a very wide departure from the normal type ot dentition is exhibited in the Proboscidian tribe of Pachyderms repre- sented in the present day by the Elephant alone; for whilst the incisors ol the upper j aw acquire those enormous dimensions which obtain for them the name of tusks, those of the lower are absent ; and whilst the true molars not only acquire a large size, but a remark- able complexity of struc- ture, the pre-molars are suppressed. Now the den- tition of the first known representative of this order, the primeval Mastodon, de- parted far less from the typical condition, than that of the later Proboscidians : for Fig. 80. not the merely is „ w structure of its true molars (Fig. 80) more simple, but a permanent pre-molar is found on either side of each jaw; and " species two in many incisors are Molar tooth of Mastodon. developed in the lower jaw, these being sometimes re- typical dentition among the earlier Mammals seen not merely in 4-1,7. u *• -. r° vwilivl ."-*.«,.uxxix<*Ao, j.» seen hot, merely m the herbivorous, but also in the carnivorous species of the older tertiarv strata; thus whilst, \ 85. But the passage from the more general to the more special is shown, not merely in the closer conformity of the more ancient forms, as compared with the existing, to archetypal generality, but also in the mode in which special characters are often first evolved. For it fre- quently (perhaps generally) happens, that the earliest forms of each prin- cipal group are notjhelowest ; but that they present in combination those characters which axe^ouScTto be separately distributed, and more dis- tinctly manifested, among groups that have subsequently made their ap- pearance. — One of the most curious exemplifications of this principle in the Radiated division of the Animal kingdom, is to be found in the history of the class E chinodermata ; for the group which seems to have attained a high development at the earliest period, is not that of Crinoidea, by which the class in question is most closely connected with Zoophytes, but that of Cystidea (Fig. 81), which (there is reason to believe) was much superior to this in general organisation. Now this order seems to have presented a most extraordinary combination of the distinc- tive characters of the remaining groups;* of which some appear not to have existed, and the rest to have presented a very limited range of forms, at the time when it was predomi- nant. Thus, the Crinoidea of the Palaeozoic period, though very numerous, exhibit but little variety of type; and in the complete enclosure of the body by polygonal plates, % they present a closer approximation to the Cystidea, than do the Crinoidea of the Second- ary period, in which the variety of forms is much greater. So, again, the Asteriada and Ophiurida of the Palaeozoic period appear to have represented only a small part of the forms which those groups have since included. It is Fig. 81. USBBEjTv Caryocrinites ornatus, one of the Cystidea. >bable that the true period ;t and although we are unfortunately not likely ever to obtain proof or disproof of the existence of Holothuriada, it cannot but be thought probable that they, too, were as yet absent. In the Secondary period, on the other hand, when the Cystidea had ceased to exist, we have evidence (save as to the Holothuriada, the softness of whose bodies would be likely to prevent their preservation) that they were replaced by all the orders just named; and these soon came to present a very high degree of" develop- * * The order Cystidea, as remarked by Prof. E. Forbes ("Memoirs of the Geological Survey of Great Britain," vol. ii.) seems to have been intermediate in structure between the Crinoidea, Ophiurida, Asteriada, and Eehinida; for it agreed with the first in the attach- ment of the body by a stem, and in possessing an intestine with an anal orifice ; the structure of the arms, in the species that possess them, accords with that of the second ; the division of the body into lobes, in certain genera, links it with the third ; and the enclosure of the body within a box-like shell, formed of polygonal plates, shows its affinity with the fourth. In addition, it may be remarked, the singleness of the generative orifice, is a strong link of connection with the Holothuriada (§ 40). t The genera Palcechinus and Palceocidaris, which have been usually referred to this group, are considered by Prof. E. Forbes as connecting links between the Cystidea and true Echinida, approximating most nearly to the former. GEOLOGICAL SUCCESSION OF ORGANIC LIFE 113 ment, dividing among them (so to speak) the characters possessed by the Uystidea. and na,mrino- Cystidea, and carrying these out separately as the distinctive pecu- liarities of their Fig. 82. re- mg spective types. — The earliest Bivalve Mol- lusk yet discovered, belongs to the exist- genus Lingula, (Fig. 82) ; which, while essentially Brachiopo- dous in structure, has no shelly frame- work, like that of the typi- cal Brachiopods, for the attachment of its arms, these being free throughout ; whilst, on the other hand, its mantle exhibits plait- jjMaiflBUBJaajj. limwY Lingula anatina. Fig. 83. Fig. 84 B II ■ ill l. t»-* - ! | [ . * |lf(J;l f m Mill i m *» M 1 Shell of Nautilus pompilius, cut open to show the chambers and the siphon. I Orihoceratite. a, Exterior ; b, Sect. showing the chambers and siphuncl I ion e. I ■ 114 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. that, whilst more elevated, in regard to its respiratory apparatus at least, than the Brachiopods which subsequently make their appearance, it is Among higher. so in virtue of its possession of Lamellibranchiate characters, the higher Mollusca, again, we find that a prominent place in the earliei formations was occupied by the group of Tetrabranchiata, including the Nautilus and its allies (Figs. 83, 84) j which presents the lowest develop- ment of the distinctive characters of the Cephalopod class, and which has much in common with the testaceous Gasteropods. Now there is no evidence of the existence of the higher order of ' Dibranchiate' Cephalo- pods, at that early date in the Palaeozoic period at which this order had acquired an extraordinary multiplication and variety of forms; and so far it might seem that we have a progression from the lower to the But the paucity of remains of typical Gasteropods, at the same period, is almost as remarkable ; and some of those forms which are most abundant (e. g. Euomphalus and Bellerophon) present indications of close proximity to Cephalopoda. So that it would seem as if the Nautiloid type is really to be regarded as having occupied the place, at that period, not merely of the order above, but also (in part) of the class below ; its decline and almost complete disappearance, during the Secondary epoch, being coincident with the multiplication of forms of the more typical Gasteropods, and of the higher Cephalopods. — Again, among the Fishes which were the earliest of the Vertebrated inhabitants of the globe, we find a remarkable assemblage of characters ; some of them presenting, in the extraordinary development of the dermo-skeleton, and in the softness and probably rudimentary condition of their vertebral skeleton, an evident ( leaning towards the In vertebrated series ; whilst others seem to have foreshadowed the class of Reptiles, an approach to which is presented, not merely by the Sharks and their allies, but by the Sauroid tribe of Osseous fishes, which was extremely abundant towards the end of the Palaeozoic period. The Cephalasjrids of the Devonian formation (Fig. 85) were not ». Fig. 85. Cephalaspis Lyellii, as seen from above, and from the side. merely remarkable for the extraordinary development of their dermo- skeleton, which has been mistaken for that of a Trilobite, but also present, GEOLOGICAL SUCCESSION OF ORGANIC LIFE. 115 to +w* A g , assiz has P oin ted out, certain characters of approximation o tne tadpoles of Batrachia ; the breathing organs and the chief part of ne alimentary apparatus having been aggregated, with the proper viscera of iht fi G ^ J \ m ^ enormous cephalic enlargement, while the rest ol the trunk, which dwindled to a point, seems to have been set apart for locomo- tion only. The Ctenoid and Cycloid orders, which (on a review of the whole class) may be undoubtedly considered as comprehending the most typical Fishes, did not make their appearance (so far as can be determined from the evidence of their fossil remains) until the Creta- ceous period. — Turning to the breathing Yertebrata, Fig. 86. we air- find , again, . . _ „ that during the Secondary period, this series was chiefly represented by the class of Reptiles, which then attained its greatest importance, and included groups which represented Fishes, Birds and Mammals respectively ; thus having a more general character than the class at present exhibits. These groups sub- sequently gave place to the more special Skllll „ f Tnh forms, which carry out most exclusively *-*«*** tilLif lff^n? t P t Al \ d When We look at the earliest for ™ of Rep- verremaXnlT V ^ an J co g^nce, we find them to present lZd2™fvK° mh r tl0nS ° f 5 e characters which are now distri- the TriZri ?• gr ° UpS - TlmS > the ^hrinthodon (Fig. 86) of s rucw Itrf ° n ' apPear£ \ t0 have W essentially Batrachian in its fracture, but to have possessed some characters of the Crocodilian order. Fi«. 87. Skull of Bhyncosaurus. The same formation contains pp™*^* ^ +i, r>i /^,. which while essentially sZ^SZ genet SeTfS £f 8?) ' 5Tt™ ' ^ Pr ° tab !f mM> y ° thCT chattel e7t^«l e pZ the same or a somewhat anterior epoch, we have the remains 7Z m >» ■■ m~m ■ 1 I ¥ ! ■ ? 116 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. Dicynodon; which seems, along with Chelonian, Crocodilian, and Saurian characters, to have possessed the peculiarly Mammalian feature of a pair of tusks growing from persistent pulps. So, again, the Ichthyosaurus, whilst essentially Saurian in its osteology, had not merely the bi-concave vertebra? of a Fish, but paddles of a Cetacean type, and a peculiar stern o-acromial And the Plesiosaurus, apparatus resembling that of the Ornithorhyncus, which is spoken-of by Cuvier as the most c heteroclite' of all the fossil animals known to him, possessed the teeth of the Crocodile with the head of a Lizard, a neck that resembled the body of a Serpent, the ribs of a Chameleon, and paddles still more decidedly Cetacean. — In the early his- tory of the class Mammalia, so far as known to us, the same general plan may be traced. The only order that is distinctly recognizable by the remains preserved in the Secondary strata, is that Marsupial which has much in common with the Oviparous Vertebrata. Near the commencement of the Tertiary epoch, remains of Pachydermata are abundant ; but these were for the most part different from those of the present epoch, containing combinations of characters which are now dis- tributed amoag several distinct families, and presenting also a closer approximation to the Herbivorous Cetaceans on the. one hand, and to the Ruminants on the other, than is exhibited by any existing species of the order. A most curious combination of characters is presented by the extinct Toxodon; which had the incisors of Hoclentia, many of the cranial characters of Cetacea, the molar teeth and massive stature of the Gravigrade Edentata, with a general conformation which seems referable Fig. 88. Skeleton of Mglodon. to the Pachyderm type. The Gravigrade Edentata, of which the Mylodon (Fig. 88) is a characteristic example, themselves afford a most interesting GEOLOGICAL SUCCESSION OF ORGANIC LIFE. 117 " f !S S S T neral P rinci P le i fOT ***** essentially allied to the to ot W !°t v m • , g ^ eral stmcture > they had not only affinities of ttStfn + fi! 0nS ° f the ,? deiltate ord ^ but also presented links extinct.- maSS1Ye Pacl ^ derms - Thi s group is now entirely mustb^Z^^i* 11 . 6 Ge °! 0giCa 1 1 ! aist ° r ^ 0f the Stable kingdom, it So^oS™if a T kn / Wl6dge i \ StiU ™y imperfect; if co^e- Sficatiot n tb ^ 1 C f e l m Which the internal str ^re and SSSr/^ ear l ie I + Pknts „ W J*** Preserved, in a condition that allows of the exact determination of their characters and affinities So far as our present information extends, however, it is full v in harmonv with the above doctrine ; the characteristic Flora of the o££fi£^S appearing to have been chiefly composed of Conifer*, wlich tn S tSe a connecting hnk between the iWerogamia and CryptogamTa ^ and G ? these Conifers, while some may have been nearly allie^tofSn/forms the great majority {Sigillariai, Lepidodendra, Galaxies, &c) appear to have presented such a combination of the characters of the CoiKrfth those of the higher Cryptogamia, as no existing group exhibits ^ 87. feo far as at present known, therefore, the general facts of Pal*> life life of it- qnrl +bo+ ,v +i • — . U1 5 auiscu uemg wnicn nowpeonle t, and that m the successive introduction of the several that > ™ every Paleontologist and Vegettbtes m^X g ° W Tf ° U f • the extinct *™* of A »^ls cwcufo^ of 2 L7^ n ^ P M0S + °P Wcal «****> of classification, as seem osculant or ^W&rms ; connecting together the groups ~whi<£ 1.7 to assemble round existing types, and seldom standing as It would be premature and premmptuons to assert that such « the plan Inihieh proceeded- bm+ +1™ # • • ,f^ . OUIC ™ »' vrgauie creation has iustiS tl7 ,, ? e ^eg^g/ndicatious may be thought sufficient to jusuny tlie assertion +.ba+. Q -n«i, ™„„. z, — v xn. . -i T „ .,. . 11 this view ), nTO " x. t ,. .' . "—" nw,y IUJU uv uccu trie pian. 11 this view have a foundation m truth, the development of the principle in all Z completeness must be left for the time, when Pala^ontolog^ shall po S s ess as the result of the accumulated labours of many generations (i mTv be) of ] industrious explorers, a collection of information respectil^ past distribution of Animal and Vegetable life upon our 3e i^L™ degree comparable to that to which the Natural " ' * ' S ° me time is rapidly attaining, t History r T J to Ve f W at fiS £ ta^T ,°* ^ ^ *"■ "*"^ ^ tional illustrations afforded by 'later Anato" Si ^TT^ t0 - S f forth ' with th « addi- first propounded by Von Baer in hfs t«2!T and Embryological researches, the views Thiere,'; 1828, ancl recentty ' Z^ ££%£& ' ^er Entwickelungs-Geschichte der translation, in " Taylor's Scientific Memohs/'Tssls! Y Mr ' Huxley ' S ^^^H 118 GENERAL VIEW OF THE VITAL FUNCTIONS. CHAPTER II. ■ GENERAL VIEW OF THE VITAL OPERATIONS OF LIVING BEINGS, AND OF THEIR MUTUAL RELATIONS. 88. The study of the various forms and combinations of Organic Structure, which present themselves to the Anatomist in his general survey of the Animated Creation, and the determination of the plan according to which we may suppose them to have been evolved, consti- tute a department of Biological enquiry that is quite distinct from that on which we are now to enter, — namely, the study of the changes which take place in these bodies, during their existence as living organisms : — a connecting link between, the two being furnished by the pheno- mena of Development, which strictly belong to the last-mentioned cate- gory, although, as we have seen, they afford most essential assistance in the preceding enquiry. As it should be the principal aim of the scientific Physiologist, to determine the laws according to which these changes take place, it is his first business to collect and compare all the facts of the same character, which he can draw from the most extended observa- tion of the phenomena of Life. The changes which occur during the life of any one being, are of themselves inadequate to furnish the required information : since this presents us only with a group of dissimilar pheno- mena, incapable of comparison with each other, or permitting it but to a low degree. Were we, for example, to derive all our notions of Physiology from the history of one of the simple Cellular Plants, we should obtain but very vague ideas as to the character of its different nutritive pro- cesses; since we cannot separate these from one another, so as to be enabled to investigate them apart. And, on the other hand, we should be apt to form very erroneous conceptions of the essential conditions of these processes, were we to study them only in that specialized condition which they present in the most complete Animal, and were to reason thence as to their dependence upon particular kinds of structure. It is only, then, from a comprehensive survey of the whole Organised Creation, — embracing the unobtrusive manifestations of vitality which Nature pre- sents at one extremity of the scale (as if to show the real simplicity of her operations), as well as those obvious changes which she every moment displays to us in her most elaborate and varied works (as if to display the endless fertility of her resources), — that any laws possessing a claim to generality can be deduced. 89. In every living Organism of a complex nature, we can readily distinguish a great variety of actions, resulting from the exercise of the different powers of its several component parts ; and these actions are said functions Muscle cation of a stimulus; and that of a Nerve, to receive and to convey sensory or motor impressions When . ANALYSIS OF FUNCTIONS. 119 howe ver as a whole, we perceive that these changes may be ciatprl W^ ~ • • , r wiese unaiiges may oe asso- SrouB cot S° UPS? f aCCor(W wit * ^eir relations to each other j each CI tw ? g M1 assembla S e of actions, which, though differing purpose To £*' ""^ * f ^ S ° me dominate and important appHed but n of"- 6 g + i° UpS ° f aCti ° nS ' ak0 ' the name of Fusions is ti ' Functio^f * r aia " SenSe aS bef ° m Thus when we «P«k of acLi^rol?^™^ ^ A mply the assemb %e of those separate expoSn^t t t W * t^^ *" f^ * ° f tlie ^tritious fluid, by a f?° "l g X * t0 ib f atmosphere, or to the gases diffused through water so as to effect a certain change in its composition. Simple as tiffs option appears many provisions are required in the higher Z*22£vZ at^onsfs wT C ,, • * *" ?* ^ ' W mSrt * ^^S^ race, consisting of a thm membrane permeable to gases ■ on on e Jh „ nf which the blood m ay be spread out, while the air if fc<^T^t£ other Secondly there must be a provision for continually renewing the mass of°Tt ^ f iCh ^?^ to tMs SUrfaCe ' * ° rder that Te whole mass of it may be equally benefited by the process And tbiJnJ +T concur purpose ¥nw Tf wa ™ 1 xt I J ^^^ uu ^ same iunaamental the ^Function o^ ^"Tf^ "*"** of phenomena, which constitute the latfPT- ^ a, ^ ~~t— *^ iui un« purpose 01 facilitating these ; and exactly proporttZl rTfl, art ^ C f % ^^ With a de S ree of s ™*ss oart of + w? . . the com P let eness of the imitation. The essential ^ ^IJZ^a tTi the ^tf ° f ^ bl0 ° d > a11 the othei change essentir s harTnf L it S v^ "" ^w^ in * mUSt be re S arded as --- of it Thus fh! Zlt r 7 b y. c ^tributmg to this-the real constituent effected h v tl * Nations m the capacity of the chest, which are renewal of ^ aCtl ° n + S ,° f * he . dia P tr agm, and have for their object the reaUv a part of T?% "J £ T^ With the aeratin S «^ are are LVf +J factions of the Muscular and Nervous systems: and teLT 7 f ° Cia ? d Und6r that ° f Res P ir ation, on account of their obvious Sf 7 M^ ltS 6SSential P Ur P° Se - ThQ y ha ve no share in the pT duction of the aeration of the blood, except by supplying its condffiW and if these conditions can be supplied independently of them t b . 1 > t^part of the function will be performed L when thf/ weTe ctcS 90. By an analysis of this kind applied to the other Functions, similar it*;^ Mood to the brain soon parses held, those movements eease. But, if the chest *.^^^?T H, T tB ^ with " by pressure, and these alternate morements beLffi^ J t ^ff' and 6mptied *&** latum of the blood through the ha^X^^!^ 1 ^ ^ ^ f"^ 6 the ci ™- whole train of vital actions may be ^X^S^n^^ v ^^ there > the movements result from a cause primarily Zl SS . ' * ' Ce f atl0n ° f the re spiratory V imaniy aflectmg the nervous system— as when narcotism np 120 GENERAL VIEW OP THE VITAL FUNCTIONS. * i ► resembles swallowing. conclusions might be arrived-at respecting their essential character ; for it will appear in every one of them, that some of the changes which are thus grouped together are essential, whilst others are superadded. But these conclusions do not possess the same certainty as if they were founded upon a broader basis; nor are they so easily attained. For, to revert to the instance just quoted, observation alone of the vital phenomena of the lower animals, will reveal what could only be determined in the higher by experiment. Until an experiment (the insufflation of the chest) had been found successful in continuing the aeration of the blood, it could not be certainly known that the respiratory movements had not some further share in the function, than that of mechanically renewing the air in apposition with the circulating fluid. But when the conditions of the function are examined in the lower animals, it is found that these are varied (the essential part being everywhere the same) to suit the respec- tive circumstances of their existence. Thus, many Reptiles, having no diaphragm, are obliged to fill the lungs with air by a process which In Fishes and other aquatic animals, to have introduced the necessary amount of the dense element they inhabit into the interior of the system, would have occasioned an immense expendi- ture of muscular power; and the required purpose is answered, by send- ing the blood to meet the water which is in apposition with the external surface. And in those simple creatures, in which the fluids appear equally diffused through the whole system, their required aeration is effected by the mere contact of the water with the general surface ; the stratum in immediate apposition with it being renewed, either by their own change of place, or, if they are fixed to a particular spot, through the means they possess of creating currents, by which their supply of food also is brought to them. And, going still further, we find the essential part of the function of Respiration performed in Plants without any movement whatever ; the wide extension of the surface in contact with the atmosphere, affording all the requisite facility for the aeration of the circulating fluid. — It is by such a mode of analysis, then, that we are most certainly enabled to distinguish the essential conditions of vital phenomena, from those which are superadded or accidental ; and it is this which will form the subject of the greater part of the present Treatise. 91. When we examine and compare the several Functions, or assem- blages of Vital Actions grouped together according to the principle just set forth, we find that they are themselves capable of some degree of classification. Indeed the distinction between the groups into which they may be arranged, is one of fundamental importance in Physiology. If we contemplate the history of the life of a Plant, we perceive that it grows from a minute germ to a fabric of sometimes gigantic size, generates a large quantity of organic compounds which it appropriates as the mate- rials of its own structure, and multiplies its species by the production of germs similar to that from which it originated ; but that it performs all these complex operations, without (so far as we can perceive) either feeling or thinking, without consciousness or the exertion of will. All its vital is induced by poisoning with opium — and the blood be, in consequence, stagnated in the lungs by the want of aeration, this change, so essential to the continuance of vitality, may be prolonged by artificial respiration, until the narcotism subsides. FUNCTIONS OF ORGANIC LIFE. 121 ■ operations, therefore, are grouped together under the general designation ol * unctions of Organic or Vegetative life ; and they are subdivided into tliose concerned in the maintenance and extension of the structure of the Nuti 4.-L ' . — — *■"— ' "^ ■>■■ wviwwioj amu. uuuat; UU WillUXl wie perpetuation of the apecies, by the Generation of a succession of indi- viduals^ is due.— The great feature of the Nutritive operations in the -riant, is their constructive character. They seem as if destined merely tor the building-up and extension of the fabric ; and to this extension tnere appears to be m some cases no determinate limit. It is important to remark, however, that the growth of the more permanent parts of the re.Iw I" f^ym^-totyj the successive development, decay, and lenewal of parts whose existence is temporary; the growth of the dense durable, but almost inert woody structure, being dependent upon the continual production of new leaves, composed of a soft, transitory but active cellular parenchyma.— Sooner or later, however, the life of the individual must come to an end ; and the race itself must become extinct were it not for the special provision which is made for its continuance in' the Generative function. This consists in the evolution of germs which becoming detached from the parent, are able to support an independent existence, usually at the expense, however, in the first instance, of nutri- ment in some way provided by the being that gave them origin: they gradually become developed into its likeness, perform all the vital opera a SSJ^SS? 1C ° f * aild " ^ W ° rigiGate a MW "IT by 92 Now, it may be observed, before proceeding further, that there is tons d t,r G ° f ^r™ W T the Nutri " Ve and ^e GeSv functions, the one set being executed at the expense of the other The futritivr T aratU 5 d6riVeS ^ materials 0f its °P e - ti °»s through the StSSrt l d "f ent1 ?^ d6pendent Up ° n * ** the continuance of ws activity If, therefore, the generative activity be excessive it will des W ^f r° ff fr r thG ^f at ^ — Portioro e fTe\wn thte th^ nZZTf T"' ** ™ 7 - b ° ™ iversa % observed that, wneie the nutritive functions are particularly active in supporting the ^nckv^dual, the reproductive system is in a corresponding l^de by s?, e i~T d T ""i J*™' ? th ° Alg "' ^ di ---on? attZ ed by single plants exceed those exhibited by any other organised beino- • and m this class the fructifying system is often obscure, and sometimes even undiscoverable. In the Fungi, on the other hand, almost the whole plant seems made up of reproductive organs ; and as soon as these have brought their germs to maturity, it ceases to exist. In the Flowerinff-PW moreover, it is well known that an over-supply of nutriment will cause an evolution of leaves at the expense of the flowers, so that what actuaUv would have been flower-buds, are converted into leaf-buds ; or the parte of the flower essentially concerned in reproduction, namely the stam^" # v ^riz:^ m ^^zz^ t as in ^F d = florets of the < disk' in compost ^^^'^^ verted into the barren but expanded florets of the 'ray.' And th I gardener who wishes to render a tree more productive of fruit, if obliged to restrain its luxuriance by pruning, or to limit its supply of food W trenching round the roots.-The same antagonism may be wi ne S se d t ■m^^mi ^» * M » * r H 122 GENERAL VIEW OF THE ^ITAL FUNCTIONS. the Animal Kingdom; but as a third element (the sensori-motor appara- tus) here comes into operation, it is not always so apparent. It appears to be a universal principle, however, that during the period of rapid growth, when all the energies of the system are concentrated upon the perfection of its individual structure, the reproductive system remains dormant, and is not aroused until the diminished activity of the nutritive functions allows it to be exercised without injiiry to them. Thus, in the Larva condition of the Insect, the assimilation of food and the increase of its bulk seem the sole obj ects of its existence ; its loco- motive powers are only adapted to obtain nourishment that is within easy reach, to which it is directed by the position of its egg, and by an unerring instinct that seems to have no other end. The same is the case, more or less, with all young animals ; although there are few in which voracity is so predominant a characteristic. In the Imago or perfect Insect, on the other hand, the fulfilment of the purposes of its generative system appears to be the chief and often the only end of its being. The increased locomotive powers which are conferred upon it, are evidently designed to enable it to seek its mate ; its instinct appears to direct it to this object, as before to the acquisition of food; it now shuns the aliment it previously devoured with avidity, and frequently dies as soon as the foundation is laid for a new generation, without having taken any nutri- ment from the period of its first metamorphosis. In the adult condition of the higher Animals, again, it is always found that, as in Plants, an excessive activity of the nutritive functions indisposes the system to the performance of the reproductive ; a moderately-fed population multiply- ing (ceteris paribus) more rapidly than one habituated to a plethoric condition. 93. On analysing the operations which take place in the Animal body, we find that a large number of them are of essentially the same character with the foregoing, and differ only in the conditions under which they are performed ; so that we may, in fact, readily separate the Organic func- tions, which are directly concerned in the development and maintenance of the fabric, from the Animal functions, which render the individual conscious of external impressions, and capable of executing spontaneous movements. The relative development of the organs destined to these two purposes, differs considerably in the several groups of Animals, as we have already in part seen (chap, i.) whole much more ' vegetative' than The life of a Zoophyte is upon the animal' : and we perceive in it, not merely the very feeble development of those powers which are peculiar to the Animal kingdom, but also that tendency to indefinite extension which is characteristic of the Plant. In the perfect Insect, we have the opposite extreme ; the most active powers of motion, and sensations of which some (at least) are very acute, being combined with a low deve- lopment of the organs of nutrition. In Man and his allies, we have less active powers of locomotion, but a much greater variety of Animal facul- ties ; whilst the instruments of the organic or nutritive operations attain their highest development, and their greatest degree of mutual depend- ence. We see in the fabric of all beings in which the Animal powers are much developed, an almost entire want of that tendency to indefinite extension which is so characteristic of the Plant ; and when the large amount of food consumed by them is considered, the question naturally FUNCTIONS OF ANIMAL LIFE, 123 arises, to what purpose this food is applied, and what is the necessity 101 the continued activity of the Organic functions, when once the fabric ias attained the limit of its development. —The answer to this question ies m the tact, that the exercise of the Animal functions is essentially c ^tructive of their instruments ; every operation of the Nervous and vxuscular systems requiring, as its necessary condition, a disintegration oi a certain part of their tissues, probably by their elements being caused vL^L wlt Vl yg f- Tb f dura ti as * those of &***, cells b v who!! ^ ^ °r + UCt !r' d6Cay ' e ™^on, and renewal, of the a cL^ , f i mstram f* al % they are effected ; which altogether effect elude ll ^ ^ P . e n f traba ° f the s y gtem ' * **& a manner as to elude observation, except that of the most scrutinizir " ' " £T™ m + brm S^g thls int0 view > that the Microscope has rendered one of its most essential services in Physiology. 95 The regular maintenance of the functions of Animal life is thus entirely dependent upon the due performance of the Nutritive operations ±sut there also exists a connection between the Organic and Animal functions of an entirely reverse kind ; for the conditions of Animal exkt ence render the former in a great degree dependent on the latter Tn the acquisition of food, for example, riie Animal has to male use f its obtain without any such assistant M £*** ^ ?? SUpP ° rt ' can along the alimen Jry oL7TZ%o^T' ^ ^^ ° f *" f °° d in which the Nervous and Mw ' T^™* a sen ? ° f °P era tions, th* +wo <^™ / • i iVlusCTllar systems are together involved J tne two extremes (simple mmmW «™+ r,-, , ■ , VBU at through the neater part 'ofTS I G ; nt ™ tlht J hemg ^ne employed th* X greater part ol the intestinal canal); and thus we see +W the change m the conditions required for the ingestion of food by 124 GENERAL VIEW OF THE VITAL FUNCTIONS. Animals, has rendered necessary the introduction of additional elements into the apparatus, to which nothing comparable was to be found in Plants. The same may be observed, as already pointed out, in the opera- tions of the Respiratory apparatus. And it may be stated as a general rule, that the more exalted is the animality of any particular being (or, in other words, the more complete the manifestation of characters pecu- liarly animal), the more closely are the organic functions brought into relation with it.* 96. From what has been said, then, it appears that all the functions of the body, among the higher Animals, are so completely bound up together, that none can be suspended without the cessation of the rest. The properties of all the tissues and organs are dependent upon their regular Nutrition by a due supply of perfectly-elaborated blood ; and this cannot be effected, unless the functions of Circulation, Respiration, and Secretion, be performed with regularity,— the first being necessary to convey the supply of nutritious fluid, and the two latter to eliminate from it the impurities with which it is continually becoming charged. The Respiration cannot be maintained, without the integrity of a certain part of the nervous system : and the due action of this, again, is depend- ent upon its regular nutrition. The materials necessary for the replace- ment of such as are continually being separated from the blood, can only be supplied by the Absorption of ingested aliment ; and this cannot be accomplished, without the preliminary process of Digestion. The Ingestion of food into the stomach, again, is dependent, like the ac- tions of respiration, upon the operations of a certain part of the muscular apparatus and of the nervous centres ; and the previous acquirement of food necessarily involves the purely Animal powers. On the other hand, the functions of Animal life are even more closely dependent upon their proper pabulum, than are those of Nutrition in general : for many rowth interrupt structure instantaneously affected by a cessation of the due supply of blood, or by a depravation of its quality. 97. Yet, however intimate may be the bond of union between the Organic and Animal functions, the former are never immediately depend- ent upon the latter; although, as already shown, they generally depend upon them for the conditions of their maintenance. There is no good reason to believe that c nervous agency is essential to the processes of Nutrition and Secretion in Animals, any more than to the corresponding processes in Plants. This is a question which may be more certainly That determined by observation, than by any possible experiment. these processes are very readily influenced by changes in the condition ot * A simple illustration will render this evident. — In certain of the lower tribes of animals, whose locomotive powers are feeble and general habits inactive, the circulation of nutritive fluid is carried on nearly in the same manner as in plants ; there is no central organ for propelling it through the vessels, and ensuring its regular and equable distribution ; and its motion appears dependent upon the forces created in the individual parts themselves. In the higher classes, on the other hand, the comparative activity of all the functions, and the peculiar dependence of those of animal life upon a constant supply of the vital fluid, require a much more elaborate apparatus, and especially a central power, by which the movements of that fluid through the individual parts may be harmonised, directed, and controlled. MM RELATIONS OF ORGANIC AND ANIMAL FUNCTIONS. 125 f f e r in e e 6 I -l . »/ J •* ~-.~wa.J-j Mivtiiixuuvu. ^ CUXXVA iU liO UXX^ XXX UXXXXCliU V V^X this connection, which has given rise to the idea of a relation of depend- ence, and which prevents that idea from being experimentally disproved. n order to cut off all nervous communication from any portion of the organism— a gland for example,— so violent an operation is required ^involving no less than the complete division of the blood-vessels, on which a plexus of ganglionic nerves is minutely distributed), that it is impossible to say that the disturbance of the function may not be owing to the shock produced on the general system. Observation shows us However, that these processes are performed in the most complex and elaborate manner by Vegetables, in which all the attempts that have oeen made to prove the existence of a nervous system have signally tailed (these attempts seeming to have been only excited by an indispo- sition to admit the possibility of any vital actions being independent of nervous influence') ;— that the lowest Animals appear equally destitute ol a nervous apparatus destined to influence them;— that in the higher classes, there are many tissues into which nerves cannot be traced and which yet exhibit as much vital activity as those which are in most inti- mate relation with nerves ;— and that in their early embryonic condition greatest P o« Z m w! 1S qmte C6rtain that n0 nerves have y et be en developed. ~t w- , xr erVoug systems : but these may the instrumentality of the Muscular and in eivous *y > R . . ^ d+ill be regarded as not involving changes of a psychical character, xusm t U hiS?rhoweve rj we find the Psychical endowments of the animal evnientlv concerned fo procuring the means of its support ; and m Man, Twlioni these exist in ^heir highest perfection, the reliance upon them th^uX of his physical wants, except under the direction of his Intel- Haence -The picts of the Digestive operation may pass by simple Sudation from the digestive sac into jft«fi*** ■ -^™^ it • or they may be taken-up by vessels distributed on its walls. In the hiker Animal;, we find that this absorption is not effected by the blood- vessels afonTbut that it is partly performed by a special set of Lacteal IZrbeTi^A over the walls of the digestive cavity j these discharge le^rs Ly have taken-up, into the cu^ut * <£«tfS3l it is probable that in this ' absorbent system that function ot ££™*" t on cTmences, by which the crude material is prepared ****££* in the formative processes. There is another division of this absorbent svstem ' which extends itself through the body, and which seems destined to coffect the superfluous nutritive material that may have escaped from the blood-vessels, together (perhaps) with such as may have served its purpose in the system, and have died without decomposing so as to be Sn available i an alimentary substance ; and this Lymphatic system of vessel also, would seem to partake in the Assimilative operation, of Wl ?00 l^^^^^***** up by the absorbent process, are 10U. lhe aiimeubd.ij ._,._ _ n ^^^ ^ *iu» fchrin. This movement, Z^tolT^Z^l^^ Plants and Animals, becomes less ^ in the lowerfwhere the_ ^^^^-^^T^- reTation wtth the parts to be supplied with nourishment. Besides aftord- II a continued supply of nutrient material for the maintenance of he formative operations, the circulating system of Animals is usually the Cns of conveying to their nervo-muscular apparatus the ojygenwho e nresence is a necessary condition of its vital activity. It also serves to take un the effete matters which are set free by the _ Ivstem and to convey these to the organs provided for their elimination. The Srculation in Animals, as in Plants, is entirely independent of the lhe Circulation ux » _ + _n„,i w u ^A_ n its usual condi- < waste' of the degr will, cauiiuo uc-ijj- «"" - o. tion, is even unaccompanied with consciousness. Muscular ASSIMILATION, NUTRITION, AND DEPURATION. 127 le s e d at d m e of ,re at, less ate Ird- ,he he ose to the on. the idi- ,tus condSs of T ' T J t0 / 1Ve t0 [t the ener ^ and r& ^ritj which the mexelvT^lT^ ^^T re(luire « 95 note )> aad Ner ™s ^ncy menS syZns ***** *** °^ ° perati ° nS ° f the COT P oreal and mii? 1 " ^ 6 alimentai 7 materials first taken up by the absorbent process must undergo various changes by Assimilation, before they can be "ntro' duced into the composition of the organised fabric. There Lhti the V Tl? r ' m ^^ theSe With P recisi °m either in tne Animal ort tZ T v ^T^ The firSt Ste P whic » is Perceptible fnTe Ltte" direct ty supplied by the food; and ihJ'pS^SZS^t "" quently not required. From whichever source the^e Terived W^ nThe^Vrrto dS a a v[tT "V* ^^ -^^^Sj^ Wet iru^Thicl they ^ wf ^oT^LI^^ ^ stream, or perhaps by both • for wp fi3 ft ■ li float mg m the change in ultimate comm^i™ iSS- W ' WltW an ^ detectable marlfthe transSfon fflS^S^ FT?? ^^ wHch the body, towards the To^^ti Wwf "fe ^ 7T* ^ the Plant, and the 'liquor sanffuimV nf g +i! ? • i protoplasma' of from mere admixtures of LT g X ! Amma1 ' are ver ^ differe *t pounds, but show7ca Dac itWn.f ' ^ter and other organic corn- possess hence their rZ^JZ TT g 0r ^ d > w ^h these do not Plastic U,tZTl^Z^t ] ^^ted as organisalle or fabric are developed aT ^17^ ' ^ mdlvidual **™ of the ments of each S deri wT I ^ pi "° CeSS ° f ^«™; the ele- their composition ma^ ^reaunl ™ * ^ *** POrti0n Which through the NervSvsteTbv !! 1S P r ° CesS f fenced in the Animal, fabric ; but it doelnot seem to ^ COnd f? nS ° f the mind « of the genera It is of course ^^£^£^1^ 98 > and cannot long continue if X oL^f- th t contmue d supply of blood, there is evidence that it ml ™ °T + 10U be brou g*t to a stand ; still contained in the vessels tf J£+ .ft* Vt!^' at the 6X P ense of the blood 102. In order to Ir^Lt ?£ ' G f ? G CUrrent has ceased to flow. may be comprehended in the general term S^^l ST geS tutmg the special function of Excretion Tht L ' the 1 former consti- to the welfare of the system, thanTs the ab^tfon "o'f ? ^ >!**«* m proportion to the complexity of the sWW a ?S , aWnt > and actions, do we find a miltipU^tioiwrf 2 ' and t0 the ^^rsity of its variety in their products The elL^v™™^ 8 ° rgans ' as wel1 as a Exhalation, and of carbonic acirl w p*- • ° su P erfluou s water by in all living beings; and in 2nim a k?'TT' ^ C ° nStant > Wev 4 azotized compound to be nearlv T * & ^ d an excreti on of a highly have no more'immediate ^ ZXV ^^ ChangeS ^ *> those of nutrition; and they wuhSL. + T ^^ than have v wm take place, to a certain extent, after the 128 GENERAL 1 final extinction of the animal powers. Wherever a proper Circulation exists, however, they are most intimately dependent upon its mainte- nance, and soon come to an end if it ceases ; but it is probable that, in particular cases, they are kept up by the capillary circulation, when the general propulsive force is no longer acting. 103. The function of Nutrition is exercised, not merely in renovating and extending the single fabric which first originates in the germ, but also, throughout the Vegetable kingdom, as well as in a large proportion of the Animal, in developing parts which can maintain an independent existence, and which are commonly accounted distinct ' individuals.' Thus, in the Protophyta and the Protozoa, each new cell formed by the subdivision of those previously existing, may be regarded either as a part of the parent structure, or as a distinct individual, being capable of living, growing, and multiplying by itself. Even in higher members of both kingdoms, a like Multiplication of (so-called) individuals is effected by the development of gemmce; a process which corresponds in every essential particular with that of ordinary Nutrition, and which is no more dependent than it is upon the activity of the Animal functions. And in many Animals, in which this power does not extei^d to the mul- tiplication of parts capable of maintaining an independent existence, it suffices to reproduce entire organs which have been removed, or (some- times) to multiply them beyond their regular number. 104. The true Generative function, by which the foundation of an entirely new organism is laid, is essentially antagonistic (as already re- marked) to the preceding; for instead of consisting in the extension of the original fabric by the subdivision of its cells, or by the formation of new tissue in connection with the old, it requires, as its essential condi- tion, the reunion of the contents of two cells, which, though not differen- tiated in the lowest Plants and Animals, either from those of the re- mainder of the organism, or from each other, are distinguishable in all but these as ' sperm-cells ' and these takes place in Plants without any interference of will, or excite- ment of consciousness, on the part of the individual ; the two organs being sometimes united in the same being (as in hermaphrodite flowers), or, if separated (as in monoecious species), being brought into the required relation by external assistance, as when the pollen of one flower is con- veyed to the stigma of another at some distance, by the agency of the wind, of insects, &c. There are some Animals in which the two sets of organs are united in the same individual; and the actions necessary to bring them into relation seem no more to depend upon conscious agency, than do those which are concerned in the aeration of the blood. But in the higher classes, where the organs exist in separate individuals, the nervo-muscular apparatus excited by powerful sensations is evidently the instrument by which they are brought into relation with one another ; and in Man, where the sensations are connected with a nobler and purer passion, not only the will, but the highest powers of the intellect, are put in action to gratify it. — But even here, the essential part of the function, which consists in the fertilization of the ' germ-cell ' by the contents of the ' sperm-cell, 5 is as completely independent of mental influence, as it is in the plant or in the simplest animal. 105. The function of Muscular Contraction, to whirVh n^arlv nil +Tia germ-cells.' The concurrent action of M^HMMB FUNCTIONS OF ANIMAL LIFE. 129 sensible motions of the higher Animals are due, is one which has an important connection with almost every one of their vital operations ; although, as already explained, this connection is mostly of an indirect character. The property of Contractility on the application of a stimulus « not, however, confined to animals ; since it is possessed by many of the v egetabie tissues, and has an important relation with their nutritive processes. Nor, even in animals, is it confined to the muscular tissue. * or m the lowest tribes it seems generally diffused through the fabric, and appears to be for the most part excited by external stimuli. But in the higher classes, it is concentrated in a special texture, and is called mto operation by a peculiar stimulus, the Nervous power, which origi- nates m the individual itself. By this means it is brought under subor- dination to the Mind, and is made the instrument of changing the rela- tions between the living organism and the external world. I f\£i nni _/?_ j • i n 1 1 1-7- ^ by stem are twofold. First, to bring ,, ,. , . * o n its most extended sense, to denote the psychical endowments of animals in general) into relation with the external world; by informing it, through the medium of the organs of sensatvm, of the changes which the material universe undergoes; and by enabling it to react upon them through the organs of motion. And duaT wTtbo° r neCt ^f harm ° niSe different aCtl0nS in ^ «™ Mvf word7?f T ^cessarily exciting any mental operation. But, in the Mind Nervous animals with dltions to. 1^ aU but the Ver ^ West animals > P a * of the con- SSTS^ 7 I mai ^ ten r ce of ^ entity, 'and of the vital pendent "- S° nmm ! hm ? *?* on whi <* all the organic life is de- endowments n^f? P - n VY COm P lex % ^ extent of the Psychical forCC *f +L n SpeCie ? ° f . Am r h > m ^ their infl ^ ce °™ the con- and mte remov./f ^ $7*F ^ perCeived ' SO that ** becomes ™™ end ofww! t that WhlCh 1S P resente <* by Vegetables, the chief fabric ^frTm tL e ^ StenC : f*™™ * be the elaboration of an organised W tW elem ™te furnished by the inorganic world. In Man, the ofoS™? P rf GSSeS S* krgeSt Share 0f these capabilities, the apparatus tyiqiyi+^ *» n 7 -v W vxu.va* xv/x j-xuuic ci»t; uiia,ii to serve ior Dne maintenance of the Animal functions ; the processes of Nutrition being almost entirely directed towards perfecting his Nervo-muscular apparatus! and bringing its functions into most advantageous operation. And it is m him, moreover, that we observe the most unequivocal indications of of the ire vi t^ UenC + - ° f *5*«™» S ^tem over tie most essentia Tparts oi the vital operations ; the processes of nutrition and secretion £1™ 107. In our search for the general laws nf +V,o ¥u„i t? +• in othpr wnvrl« ft™ +-u„ s-="cxd, A ld ,ws oi the Vital functions, — or in utner words, tor the general man nf Vi+ol A „+• :* i n \ . ' great advantage from keening £ fi • • Actl ^'^ we sha11 der ^e W« /r «L ^ZSz fo ft 7 7 m V16W ' that Y ° n Baer ' S law of regard to the fuldZt^otJTl ****"* t0 5f g °° d aS ^ in uiaracter oi organs, as with respect to their ■K Prof. Alison's "Outlines of Human Physiology," p. 13. K 130 GENERAL VIEW OF THE VITAL FUNCTIONS. > r ■ structural and developmental conformity ; as may be seen in proceeding from the lower to the higher forms of organised being, and in following the successive stages of development of any one of the higher organisms. (Xi we compare the forms which the same instrumental structure pre- j sents in different parts of the series, we shall always observe that it 1 exists in its most general or diffused form in the lowest classes, and in j its most special and restricted in the highest ; and that the transition from one form to the other is a gradual one. The function, therefore, _ - which is at first most general, and is so combined with others performed by the same surface as scarcely to be distinguishable from them, is after- wards found to be limited to a single organ, or to be specialized by separation from the rest; these also, by a similar change, having been rendered dependent on distinct organs. H * organism (so to speak) comes to be accomplished by a ' division of labour amongst its several organs, each of which is adapted to execute its par- ticular share with a measure of energy and completeness proportionate to the speciality of its development. 108. Thus, to refer again to the provisions existing in different Plants for the A bsorption of fluid ; we find that this action is performed by the entire surface of the Protophyta, which may be thus said to be all root; as we ascend through the series of Cryptogamia, it becomes more and more limited to certain parts of that surface ; until in Flowering plants it is chiefly performed by its special organs, the ' spongioles,' or succulent ex- tremities of the root-fibres, which draw-in fluid far more energetically than any portion of the general surface can do. And so in the early development of Phanerogamia, the germ absorbs by its whole surface the nutriment in which it lies embedded ; it continues still to absorb by a part of that surface, even after the expansion of its plumula and the ex- tension of its radicle into the soil ; and it is not until the store laid-up by the parent has been nearly exhausted, that the proper root-fibres are evolved, which are henceforth to be its special instruments of imbibition. With equal reason the simplest Protophyta might be said to be all leaf; their whole surface taking part in those functions, which are restricted, in the higher forms of Vegetable organisation, to the foliaceous appen- dages. Considered only in reference to its vital operations, therefore, the simplest Plant differs from the most complex, principally in this, that the whole external surface of the former participates equally in all the operations which connect it with the external world, as those of Absorption, Exhalation, and Respiration, — whilst in the latter we find that these functions are respectively confined to certain portions of the surface. However distinct, therefore, the roots and leaves of a Vascular plant are from each other, they both have a functional analogy with the same simple membrane of the lowest species of Protophyta. 109. Where the greatest degree of specialization of function presents itself, the particular arrangement of the organs will have reference to the general plari of conformation, and to the circumstances under which the being is destined to exist. Thus, whilst the absorbing organs of Plants are prolonged externally into the soil, they are usually distributed in * This principle has been frequently and forcibly dwelt on by Prof. Milne-Edwards ; who has most fully developed and illustrated it in his " Introduction a la Zoologie Grenerale," Paris, 1851. SPECIALIZATION OF FUNCTIONS. 131 S Animals upon the walls of a cavity fitted to retain and prepare the food. « 1 to Same fundamental unit y exists ; and the spongiole of the vas- cular i-iant, and the absorbent villus in the Animal have preeiselv the same essential surface with the membrane which constitutes the digestive cavity of the Hyd of a. It may, then, be enunciated as a he whole animated Creation, the functional ,.„„,. ,7 7 .- _■. _ T % possess in common, remains the same: amZr r m ?h C \ ^ <^fer « m«m/^, varies with the general plan upon winch the being is constructed. The latter part of this relw 7 T^ared more intelligible by another illustration. The rfn ^ 7 ? f Ce ,° f . Pkn * S 1S alwayS P rolon g ed externally ; as they have rlr a Tnf f m * r ° 1 dUCmg ^ **? Ca 1 tieS ' and ° f Acting that constant renewal of it which is necessary for the aeration of their nutrient fluid lne same is found to be the case in nearly all aquatic Animals, the gills oi which are evidently analogous to the leaves of Plants. In terrestrial Animals, on the contrary, the respiratory membrane is prolonged inter- The y diffL a .tt vV m * vt ° r "fl e ?° Sing a large a ^ nt * f s ^«e. Ihe different cavities which we find adapted to this office, are all lined by a membrane which is either derived immediately from the external surface as m Insects, terrestrial Mollusks, &c, or from that invers on of it which forms the digestive cavity, as in Vertebrata ; zndZZem- thToVlT/lI^ ^ cW % - instrumentally the same, alth"g1i ofterStil^ t f m " ? Pai t ^ ^ homolo S ^ ^ose of one kind Sr? ™ m S , m a rudimentary state, where another is fullv developed fl^m:^^ T nd "TT ° f thG ™ k > iUustratLn^K organs comirr if ""ft* *? fanctional cliara ^r of the different in this p kcT ; ^ * need n0t ' tWore > be fcther dwelt-upon eiataion^ * ^ f the , m ° St C ° m P lex W § s > «* %*" general Llf™ t f I?™ pr ° Ceeds S ° far > as to ^capacitate the more Cn Ti for ^ S r e P art * *'> for observation of the knowledge ^of tnofc Ctv ^ ° f ^^ h ^ leads to the namelvlLf r:^,i a I' ^i!.!T*^ -P-ssjon of this fact; different functions off will /9/v» a «7 • 7 r7""l ' *""«»*> ™ore or cess, tfie primitive community coTiousTv1n°7T?^ cha i aCterized U * AS this P rinci P le > als °> ™ be mS! + l 7 lUus . trated in subsequent chapters, it is unnecessary here to do more than point out its mode of application ; and we shall ajain refer to the function of Absorption for this purpose. As, in the simplest rw homogeneous beings, the entire surface^ participates equallv ?n the aTnf imbibition, so m the most heterogeneousf every Wt o 'the IZ^rT' some capacity for it; since, even Si the highest Plants ami AV ? T common external integument admits «f S *„ 4 mmals > th e interior of the s Y stem ^0^11^ 1 & P f Sage ° f fluid into ^e channels is defident In the tl "^Z ***"** hj the Usual lowest Animals, the font W oHwT"* ™ ^ **** WWlst ' in the entire surface, ihere IZ the ht W ' T eqUa% perf ° rmed ^ the organs, to each of which some 3T? J ^ ^ a PP aratus of Glandular ° ' . , , 7 SOme s P ecial Vision of that function is assigned • Jo*i:i^%7 mm ™ '° nU ^ of Function,' in the .'WU^JM^fcJ k2 T ■ ~- 132 GENERAL VIEW OF THE VITAL FUNCTIONS. elementary structure only in the peculiar adaptation of each to separate a particular constituent of the blood, it is in conformity with the law just stated, that either the general surface of the skin, or some of the special secreting organs, should be able to take-on, in some degree, the function of any gland whose duty is suspended ; and observation and experiment fully bear out this result, as will hereafter appear (chap, ix.) CHAPTER III. ♦ OF aliment, its ingestion and preparation. 1. Sources of the Demand for Aliment. 111. A ll Vital A ction involves a change in the condition of the Organ- ■The vital activity of the Plant ised Structure which is its instrument. is chiefly manifested in its increase, development, and reproduction; in the multiplication of its component cells, in the metamorphoses which these cells undergo, and in the formation of germs which are destined to be cast off by it, and to originate new organisms elsewhere. These operations can only be performed, however, when the plant is supplied with such alimentary substances, as can be converted by it into the proximate own them, as the pabulum at whose expense their growth those simple Cellular Plants whose In structure is nearly homogeneous throughout, every act of cell-multiplication conduces directly to the growth of the entire mass ; since the new cells thus produced remain as constituent parts of it, so long as the organism holds together. In the higher tribes of Plants, however, this is not the case ; for we find, as we have seen, that certain organs are periodically developed, the term of whose existence is comparatively brief, so that they only form constituent parts of the structure for a short time, being cast-off as soon as their term of life is over, to be replaced by another set possessing attributes of pre- cisely the same kind. It is, however, through the agency of these tem- porary organs, — namely, the leaves and the flowers, — that the means of increase and reproduction are provided for the more permanent parts of the organism, which could neither grow nor regenerate its kind without their agency. Thus it would seem to be a general rule, that wherever true woody tissue (the production of which seems to be the highest exer- tion of the vital force of Plants) is produced, the Plant shall be furnished with a set of organs, whose peculiar function it is to prepare and elaborate its materials, accomplishing this with such vigour and activity, that their own vital energy is soon exhausted, so that they die, and are cast-off in a state of decay ; and thus the most permanent portion of the higher vege- table fabrics is built-up through the instrumentality of the most transient part of their organisation, the growth of the wood-cells being entirely dependent upon the vital activity of the leaf-cells. So again, in the higher Plants, we find that instead of that simple liberation of the o- en e- SOURCES OF DEMAND FOR ALIMENT. 133 rative products from the interior of certain cells, more or less distinctly set-apart from the general structure, which is the common method of uctjiication m the lowest, a complex apparatus is provided for their Tni KTv n ii i that tMs a PP aratus > -consisting in fact, of elements which ught be developed into leaves, and which, though metamorphosed into uLe parts of the flower, still present an accordance with the general laws i ieat-growth,— is itself of yet more transient duration than the leaves, 1TI ca ^-° ff /f. soon as the germs which it has brought into existence ««e capable of living separated from the parent structure. Further we nave to note, that it is one part of the office of the apparatus of fructifi- cation, to prepare and store-up a supply of nutriment for the early de- velopment of the germ j and this store, which makes up the chief bulk of % ^seed, is derived from the materials provided by the roots and leaves oi the parent -plant. 1 12. Thus, then, if we look at the sources of demand for Aliment in the V egetable Organism, we shall see that they may be reduced to the lollo wing heads— I. The extension of the individual fabric, b y the multi plication and development of its component parts. These may all resemble frs^i^^^^.K- *» -ewith ^ be § p^-tPTi+- m. +w,™ + i 1 -"'"'"•jf JU -^* ,u -"" «" »u uimvax unlimited extent , or they may take-on a development which differentiates them ofThe 6 YafT' PI ^X WUht S ° me <" the W00d ^ *™*» - th" £Z ot the Vascular Plants) possess considerable durability, and remain as exneST- t7 ' r f™ m % for a ti ™ those that have already XK; powers !? d p r ed through their term ° f m > ^ tJ e*IZn£A?JT ? V 7 ^ r°^P. leted the sa ^ series of phases of nuSK^ f °7 ts ^^te object the extension of the more per- con wLf ofi. StrUCt ^\~ n - T } e Production of germs for the s W?r ! T 6 i T^* 1 n0t onl y Corporate in their own sub- Pwl C r"rrl° ^^^4 *"***, in the higher earlv in their ^ ^e activity 01 these Unctions is dependent upon, and in some degree determined by the supply afforded to it. Thus it is when the plant is most abundantly turnisned with appropriate materials by its roots and leaves, that it will most tend to throw out new leaf-buds, to form new wood, and thus to grow as an individual ■ whilst, on the other hand, it is when more smarmp-lv snrmlio^ TxA±in -r,,,+^:^ x j.r _, •, .„ > _ vviicii more number nf flow-buds, and develop the mcst numZs progeny^* the lite of the simpler plants therp m 9 v u ^^ ; i r^g^ny. i n ever; for so long as'thei? tis« gS and mu^l° i ™^ ^t they resist the influences which ten d £ + T* g ? m ^ply™g, so long do life of the higher pla2 a S out . £ < ^ ? ecom P oslti ™- But in the nature of some nf +w ' ^ 6 ° f waste arises from ^e temporary nature 01 some 01 their organs • tlirmo-li ,*,r™ .a,- • , t ^-"^idiy just seen, with the constructive Ittw. 5 * connect f d ^ as ™> have + ;„„ ,( w/oM/ active, not with the destructive class of n™»™ serv^S to ZT-T 1 deVe l°P ment a »<* death of the leaves bein^ub" servient to the buildmg-up of the individual fabric, and to the Z%net i 134 OF ALIMENT, ITS INGESTION AND PREPARATION, tion of the race. Thus a very large proportion of the aliment taken into the Vegetable system is directly appropriated to these purposes; the amount of carbonic acid and of other excretory matters given off during the period of growth, being very small in proportion to that of the materials introduced ; and the fall of the leaves, which restores to the condition of inorganic matter a certain amount of substance that has undergone the organising process, not taking place, until by their instru- mentality a considerable addition has been made to the solid fabric of the tree. To these processes of extension and reproduction, there would not seem to be, — at least in a large proportion of the fabrics belonging to the Vegetable kingdom, — any very definite limit. very In it, also the activity of the functions of growth and reproduction becomes a source of demand for food. But, excepting in those tribes which (like Zoophytes) multiply by gemmation, the period of increase is limited. The full size of the body is usually attained, and all the organs acquire their complete evolution, at a comparatively early period. The continued supply of food is not then requisite for the extension of the structure, but simply for its maintenance ; and the source of this demand lies in the constant ' waste,' to which, during its period of activity, it is subjected. Every action of the Nervous and Muscular systems involves the death and decay of a certain amount of the living tissue, as is indicated by the appearance of the products of that decay in the Excretions ; and a large part of the demand for food will be consequently occasioned by the neces- sity for making good the loss thus sustained. Hence we find that the demand for food bears a close relation to the activity of the c animal' or destructive functions ; and thus the Birds of most active flight, and the Mammals which are required to put forth the greatest efforts to obtain their food, need the largest and most constant supplies of nutriment ; whilst even the least active of these classes stand in remarkable contrast with the inert Reptiles, whose slow and feeble movements are attended with so little waste, that they can sustain life for weeks and even months, with little or no diminution of their usual activity, without a fresh supply of food. 114. But this waste and decay do not affect the muscular and nervous tissues alone ■ for as we have found in the Plant, that the higher parts of the structure are developed by the instrumentality of the vital activity of the lower so do we find in the Animal, that the exercise of those constructive operations, by which the materials for the first growth and the subsequent maintenance of the fabric are prepared and kept in a state of the requisite purity, involves the agency of a set of organs, which may be said to be entirely ' vegetative' in their character, and in which, as in the higher Plants, a continual renewal of the cells that constitute their essential structure, seems necessary for their functional activity. Thus all the glandular and mucous surfaces are continually forming and throwing off epithelial cells, whose production requires a regular supply of nutri- ment • and only a part of this nutriment (that which occupies the cavity of the cells) consists of matter that is destined to serve some other purpose in the system, or that has already answered it ; the remainder (that of which their solid walls are composed) being furnished by the nutritive materials of the blood, and being henceforth altogether lost to SOURCES OF DEMAND FOR ALIMENT. 135 it. Thus every act of Animal Nutrition involves a waste or decay of • T ' in its subsequent depuration. We between the amount of aliment required, and the amount of waste occasioned, by the simple exercise of the nutritive or vegetative functions in the building-up and maintenance of the animal body, and that which results from the exercise of the animal functions. The former are carried on, with scarcely any intermixture of the latter, during foetal life. The aliment, in a state ot preparation, is introduced into the foetal vessels ; and is conveyed by tnem into the various parts of the structure, which are developed at its expense. The amount of waste is then very trifling, as we may judge by the small amount of excretory matter, the product of the action of the liyer and kidneys, which has accumulated at the time of birth ; although these organs have attained a sufficient development, to act with enemv when called-upon to do so. ft _ movements begin to take place with activity, tlie waste increases greatly; and we even observe this immediately after birth, when a large part of the time is still passed in sleep, but when the actions of respiration involve a constant employment of muscular power.— In the state of profound sleep at subsequent periods of life, the vegetative functions are performed with no other exercise of the animal powers than is requisite to sustain them • and we observe that the waste, and the demand for food, are then diminished to a very low point. This is well seen in many animals, which lead a life of great activity during the warmer parts of the year, but wincii pass the wmter in a state of profound sleep, without, however, any considerable reduction of temperature ; the demand for food, instead of «rpi T^' 1S -? nly felt hj them at lon S mte ™ls, and their excretions ^1 L? • ^. educ 1 ed m amount. And those animals which become com- W? T ' e + lther J^ the influence of cold, or by the drying-up of their & d ° + n0t ' suffer f rom the mos * prolonged deprivation of food ; Decause not only are their animal functions suspended, but their nutri- ent Tr atl ° n l al 1 S ° are > com P lete abeyance; and as the continual de- o£T?°!i ^ n i 7 lucl i ^ ould otherwise be taking place in their tissues, is cnecked by the cold or by the desiccation to which they are subjected, the wnole series of changes which would be going-on in their active condition is brought completely to a stand. 116. But there is another most important source of demand for food amongst the higher Animals, which does not exist either amongst the lower Animals, or in the Vegetable kingdom. Mammals, Birds, and to a certain extent Insects also, are able to maintain the heat of their bodies at a fixed standard, and are thus made in great degree independent of variations m external temperature. This they are enabled to do, as will b e explained hereafter (chap, x Sect. 3), by a process analogous to ordinarv combustion ; the carbon and hydrogen which are directly supplied W their food, or which have been employed for a time in the compel In of their living tissues and are then set free, being made to unite with oxygen introduced by the respiratory process, and thus giving off as nTuch heat as if the same materials were burned in a furnace* And it has W experimentally proved, hat the immediate cause of death in a warT blooded animal from which food has been entirely withheld, is thf in 136 OF ALIMENT, ITS INGESTION AND PREPARATION. ability any longer to sustain that temperature, which is requisite for the performance of its vital operations. Hence we see the necessity for a constant supply of aliment, in the case of warm-blooded animals, for this purpose alone; and the demand will be chiefly regulated by the difference between the external temperature and that of the animal's body. When the heat is rapidly carried-off from the surface, by the chilling influence of the surrounding air, a much greater amount of carbon and hydrogen must be consumed within the body, to maintain its proper heat, than when the air is nearly as warm as the body itself ; so that a diet which is appropriate to the former circumstances, is superfluous and injurious in the latter ; and the food which is amply sufficient in a warm climate, is utterly destitute of power to enable the animal to resist the influence of severe cold. Again, the Bird, whose natural temperature is 110° or 112°, and the bulk of whose body is small in proportion to the surface it exposes, must consume a greater quantity of combustible material for the main- tenance of its normal heat, than is required by a Mammal, whose natural temperature is 100°, and whose body, being of much greater bulk, ex- poses a much smaller proportional surface to the cooling influence of the surrounding medium. 117. Thus we find that, in the Animal body, aliment is ordinarily re- quired for four different purposes \ the first two of which are common to it and to the Plant, whilst the others are peculiar to it. — I. The first construction or building-up of the organism, by the development and multiplication of its component parts. II. The production of germs for the continuance of the race; and, in addition, in the female, the provi- sion of the store of aliment required by these germs during their early development. period of growth, and after its attainment of its full size, notwithstanding the i waste' occasioned by the active exercise of the nervous and muscular systems. IV. The supply of the materials for the heat-producing process, by which the temperature of the body is kept up. — The amount required for these several purposes will vary, therefore, not only with the general activity of the nutritive processes, but in accordance with the conditions of the body, as regards exercise or repose, and external heat or cold. It is also subject to great variation with difference of Age. During the period of growth, a much larger supply of food is required in propor- tion to the bulk of the body, than when the full stature has been attained: but this results, not so much from the appropriation of a part of this food to the augmentation of the fabric (the proportion of its whole amount which is thus employed being extremely small), as from the much greater rapidity of change in the constituents of the body of the young animal, than in that of the adult ; which is evidenced by the large proportional amount of the excretions of the former, by the rapidity with which the effects of insufficiency of aliment manifest themselves in the diminution of the bulk and firmness of the body, by the short duration of life when food is altogether withheld, and by the readiness with which losses of substance by disease or injury are repaired, when the nutritive processes are restored to their full activity. The converse of all this holds ood in the state of advanced age. The excretions diminish in amount, the want of food may be sustained for a longer period, losses of substance are but slowly repaired, and everything indicates that the interstitial III. The maintenance of the organism both during its SOURCES OF DEMAND FOE, ALIMENT. 137 changes are performed with, comparative slowness ; and, accordingly, the demand for food is then much less in proportion to the bulk of the body, than it is in the adult. This contrast is most remarkably shown in the insect tribes, which are far more voracious in the larva than in the imago state ; many species, indeed, taking no food whatever, after their last very m ay be preparing to apply to the sustenance of their progeny. •j i . The influence of the supply of food upon the size of the indi- vidual, is very evident in the Vegetable kingdom; and it is most strikingly manifested, when a plant naturally growing in a poor dry soil is trans- ferred to a rich damp one, or when we contrast two or more individuals ot the same species, growing in localities of opposite characters. Thus says Mr. Ward,* " I have gathered, on the chalky borders of a wood in Kent, perfect specimens in full flower of JSrythroea Centawrium (Common Centaury), not more than half an inch in height; consisting of one or two pairs of most minute leaves, with one [solitary flower : these were growing on the bare chalk By tracing the plant towards and in the wood, I found it gradually increasing in size, until its full development was attained m the open parts of the wood, where it became a glorious w VT* °l five feet in elevation > and covered with hundreds of flowers " W e And, then that by starvation, naturally or artificially induced, Wants may be dwarfed, or reduced in stature : thus the Dahlia has been diminished from six feet to two ; the Spruce Fir from a lofty tree to a pigmy bush; and many of the trees of plains become more and more dwarfish as they ascend mountains, till at length they exist as mere nnderwood Part of this effect, however, is doubtless to be attributed to diminished temperature; which concurs with deficiencv of food in producing inferiority of size—The influence of variations in the supply Z IZ\ m Poncing a corresponding variety of size, seems to be less Ani^it kmgd T than in the Ye g eta ^ but this is not because meZt.f re r m T 7 J 5 ®"" ess de P e * d <^ than Plants upon a proper nieasuie of aliment. For such a limitation of the supply as would dwarf & r iant to^ any considerable extent, would be fatal to the life of an . 0n the ° ther hand > a * excess of food, which (under favour- able circumstances) would produce great increase in the size of the -riant, would have no corresponding influence on the Animal ; for its size appears to be restrained within much narrower limits,— its period ot growth being restricted to the early part of its life, and the dimen- sions proper to the species being rarely exceeded in any great degree iiven m the case of giant individuals, it does not appear that the ex' cess of size is produced by an over-supply of food; but that the larger supply of food taken-in, is called-for by the unusual wants of the system,_those wants being the result of an extraordinary activity in the processes of growth and being traceable rather to the properties inherent m the individual organism, than to an Y external a^ncie? Ti influence of a diminished suppl v of foorl iTLZ • agenC . ies - . The riority in the size of Animals if ZJ^$£ P ? ducm f a * arked ™&- early periods of growth, n which X col V ^/ff^ ^ th ° Se purelv < vesetatile ' ThZ 7+ • 11 c ? ndltl0n of the s^tem is most purely vegetative, lims it is well known to Entomologists, that, -* U On the Growth of Plants in closely glazed Cases," 2nd edition, p. 16. ■■■ . p 138 OP ALIMENT, ITS INGESTION AND PREPARATION. iuffi whilst it is rare to find Insects departing widely from the average size on the side of excess, dwarf-individuals, possessing only half the usual dimensions, or even less, are not uncommon; and there can be little doubt that these have suffered from a diminished supply of nutriment during their larva state. This variation is most apt to present itself in the very large species of Beetles, which pass several years in the larva state ; and such dwarf-specimens have even been ranked as sub-species. Abstinence _ has been observed to produce the effect, upon some Cater- pillars^ of diminishing the number of moults and accelerating the trans- formation ; in such cases, the Chrysalis is more delicate, and the size of the perfect Insect much below the average. some food, continued through successive generations, may produce a marked effect, not merely upon the stature, but upon the form and condition of the body, even in the Human race, appears from many cases in which such influence has operated on an extensive scale. Of these cases, some of the most remarkable are those of the Bushmen of Southern Africa, and the aborigines of New Holland; whose low physical condi- tion appears to be in great part due to imperfect nutrition. 119. There can be no doubt that the character of the food supplied, has an important influence upon the development of particular parts of the organism j and may thus modify its general conformation in a re- markable degree. Many of the alterations which are effected by cultiva- tion in Plants, obviously proceed from this source j and when it is known what are the particular components of any special tissue or oro-an which it is desired to augment, a supply of the appropriate pabulum will usually be effectual for this purpose. Thus the production of the Corn-grains is largely increased by azotized manure combined with the earthy phos- phates ; whilst that of the Sugar-cane is in like manner favoured by non- azotized manure combined with silex. So in the higher Animals, the pro- duction of blood-corpuscles is known to be promoted by iron, that of fat by abundance of oleaginous or farinaceous food, and even that of muscle and bone by suitable kinds of diet.— The most remarkable example, how- ever, of the influence of particular kinds of food in modifying the processes of development, is seen in the economy of the Hive-Bee. The neuters, which constitute the majority of every community, are really females with the sexual organs undeveloped, the capacity for generation beino* restricted to the queen. If by any accident the queen should be destroyed or if she be purposely removed for the sake of experiment, the bees choose two or three from among the neuter-eggs that have been depo- sited in their appropriate cells, and change these cells (by breaking-clown others around them) into royal cells, differing from them considerably in form, and of much larger dimensions; and the larvse, when they come forth, are supplied with ' royal jelly,' an aliment of a very different nature from the 'bee-bread' which is stored-up for the nourishment *of the workers, being of a pungent stimulating character. After going through its transformations, the grub thus treated comes forth a perfect queen ; differing from the ' neuter' into which it would otherwise have changed' not only in the development of the generative apparatus, but also in the form of the body, the proportionate length of the wings, the shape of the tongue, jaws, and sting, the absence of the hollows on the thighs in which the pollen is carried, and the loss of power to secrete wax. Thus NATURE OF ALIMENTARY MATERIALS. 139 in acquiring the attributes peculiar to the perfect reproductive female, the insect loses those which distinguish the working population of the mve ; and of this departure from its usual mode of development, the (liiierence in the food with which it is supplied appears to be the onlv essential condition. Nature of the A limentary Materials, 12 0. _ Amongst the general differences between the Animal and Vege- table kingdoms, none are more striking than those existing between the aliments whereon they are respectively supported, and the mode of their ingestion or introduction into the system. The essential nutriment of -Wants appears to be supplied by the Inorganic world ; and to consist chiefly of the elements of water, carbon, and nitrogen, with certain mineral compounds. The Water is partly derived from the fluid that percolates the soil, which is absorbed by the roots ; and partly from the moisture of the atmosphere, which is imbibed by the leaves.— The Carbon is principally obtained (§268) from the carbonic acid which exists in the Atmosphere in the proportion of about 0-00049 to 1; but most plants are assisted m their growth by its introduction through the roots also in all soils of moderate richness, there exists a large quantity of the remains < of organised fabrics, the upper layer of which is constantly undergoing some degree of decomposition by contact with the atmo- sphere, so that carbonic acid is formed in it. The water which traverses such a soil, therefore, will become charged with this gas; and this state of solution appears to be that in which carbon may be most Hit T Tl 7 mtroduced ™ to tte vegetable system. It seems pro- a™ i Jwf I °! ganiC T tter Whicla rich soils cont ain, ^ ™t ielf sitimf * £ /• n ? nt ^ of * he P lant > without this previous decompo- sable ™ *i 1S ^ **** those soils which afford the most steady mid Sowtl • 3 ll f ° arbonic aC ! d > are the most ^ourable to vegetable of dl^n 1S ^ may be answered > n °t merely by an admixture such 7^ P g ° rgamC i *"??' but ^ the introduction of substances, aemil gyP r m - ° r ?™ dered charcoal, which have the property of con- Z^ g ,T ?r f ld f ? m the atmosphere.-It is only within a recent period, that the dependence of all Vegetable growth upon a due supply ot JNitrogen has-been ascertained; but it is now known that, although usually existing m only a small proportion, its presence in the vegetable tissues is peculiarly important at the time of their greatest formative activity; the < primordial utricle,' which is the seat of the most active vital operations, being composed of albuminous matter, in which nitron is an essential ingredient. The small quantity of nitrogen which the usual rate of growth of ordinary Plants causes them to require, appears * It has been recently affirmed, try MM Vprdml M j tr i i. .i , , , , pound, isomeric with lignin, cellulose &c IS , Rfet, that a soluble neutral corn- result of the partial decomposition 'of the Snld S ? fr om ^rtile soils; being the these soils have been generated. The solu^S Stractur , e ? at the ex P en se of which power of taking up silica and carb onate othme ^dTSTJ m ***£ ^ * remarka We duc mg these substances into the plant Accord^ ,tf ^ ^ 0m f ^ means of ***«>- this compound may be directly 1™^^*,^ ^T^ ^° **? disco ^red it, they admit that, if not so appropriated Tt! " tnment ^ ^ wm S plants ; although (See « Comptes Kendus de KStlt V^SSSTmf ^ ° & ^^ acid ' 140 OF ALIMENT, ITS INGESTION AND PREPARATION. To to be derived from the minute proportion of ammonia existing in the atmosphere, in combination with carbonic acid; this being condensed from it in rain or dew, or absorbed in the gaseous state by porous soils, so as in either case to find its way to the roots in the liquid which they imbibe. But the growth of most plants is powerfully stimulated by an additional supply of ammonia, such as they derive from the introduction of decaying animal substances into the soil, as manures ; and the efficacy of these is peculiarly manifested in the large increase of the amount of azotized compounds, then generated by such plants (the corn-grains, for example) as naturally produce them in considerable proportion.** the fertility of a soil, then, it is essential that it yield a sufficient and regular supply of moisture, carbonic acid, and ammonia; the two latter being either attracted from the atmosphere, or evolved by its own decomposition. But however richly a soil may afford these ingredients, it will not support an active vegetation, unless it also supply in sufficient quantity the Mineral substances which Plants require. These are, for the most part, the earthy carbonates, sulphates, and phosphates, the alkaline carbonates, and silica. Most soils contain the greater number of these compounds in larger or smaller proportion; and it is mainly according to the predominance of one or other of them, that particular soils are specially fitted to support those kinds of plants in which a like predominance exists. Thus the Cerealia and Grasses require a large pro- portion of silica and of the alkaline carbonates ; Turnips and Potatoes, more of the alkalies; Peas, Beans, Clover, &c, carbonate and sulphate of Corn Water furnish phosphates, stances the^ confirmed by the fact, that not only will the simpler forms of Lichens appear on barren rocks in the midst of the ocean, increasing by absorption from the atmosphere alone, and preparing by their decompo- sition a nidus for the reception of the germs of higher orders of vege- tation ; but that many, even of the more highly organised species, will grow in circumstances where no other kind of nutriment is accessible to them. The small amount of earthy or saline matter contained in the tissues of such plants, must be derived from the atmosphere, which is known to hold such particles in suspension. 121. The only class of Plants which even seems to be dependent for its support upon matters already organised, is that of Fungi (§ 26); but it is probable that this dependence only arises from the peculiarly laro-e and constant supply of carbonic acid and ammonia, which they require According to the recent enquiries of M. Georges Yille, the mean quantity of Ammonia ^-.jained in the atmosphere is only 22*417 grms. in a million of kilogrammes, (or 0-0000000224 parts) ; the maximum quantity being 29*43 grms, and the minimum 17*14 grms. He thinks that this quantity is too small to furnish the supply of nitrogen which vegetation involves ; and maintains that plants must absorb azote direct from the atmo- spher^—an assertion, however, of which no sufficient proof is given. He found that an arti- ficial increase of the ammonia in the atmosphere to an extent of 0*0004, produces an extra- ordinary increase in the activity of the vegetative processes ; and that the plants grown in such an atmosphere contain, when mature, twice as much azote as those grown in pure air. If this treatment be employed at the commencement of the flowering season, the increased development of the leaves checks that of the flowers ; and if any flowers are produced they are barren.— (" Proceedings of the Royal Society," May 26, 1853.) NATUEE OF ALIMENTARY MATERIALS. 141 as the condition of their growth; as well as (perhaps) from their being only able to appropriate these compounds in the F nascent' state. There is no reason to believe that they can make use of organic compounds in any other than a state of decomposition, and hence it is that their great utility in the economy of Nature arises; the products of decay, yhich ^ might otherwise have poisoned the atmosphere, being converted into living and growing tissues. — Fungi present us with two curious analogies to the Animal kingdom; both resulting, no doubt, from the mode in which they receive their aliment. The large quantity of car- bonic acid with which their absorbent apparatus furnishes them, prevents the necessity of their drawing any additional supply of it from the atmosphere ; but on the contrary, like animals, they have only to get rid of what is superfluous. And again, the proportion of azotized matter contained in their tissues is much greater than in those of any other vege- table ; ^ so that their substance, if capable of being digested, is almost as nutritious as animal flesh. 122. It is a general law of vitality, that the materials of nutrition can only be introduced into the living system in the fluid state ; and although the ingestion of solid aliment by the higher Animals might seem to contradict such a principle, a little examination into the character of their YM1TVH+TTTA AVWv^iArtl^^. * "1 1 _"I I 1 I * I • Sft -* * ^ will furnished, and by which they directly imbibe their aliment from the external with of their food, and for its reduction to a state fit to enter the vessels. .necessity for these cavities arises out of the nature of the aliment required by Animals, which usually pre-exists in a form more or less S t al , S0 from the occurrence of intervals between the periods at W - lt 1S stained. Whilst the roots of Vegetables are fixed in the soil, ramify necessary carry absorbents are distributed on the walls of a digestive cavity, just as those of Plants are externally prolonged into the earth' Hyd efl as presenting us with the simplest example. It is merely a bag with one opening (a), which may be regarded as all stomach. A higher form is that in which the cavity has two orifices, and thus becomes a canal (b); and all the complicated intestinal apparatus of the higher animals may be considered as a more extended development of this simple type. That the presence of the stomach, however, is not an essential character of the Animal (as taught by some Physiologists), but is rather a special adaptation of their organism to the peculiarity of their food which may be dispensed-with under peculiar^ circum- tZT 7 ? PI T hereafter <§ 138).-The food which Is mtroduced into this cavity, is acted-upon mechanically \v nnu™1 f™ )t ? ° f tLe Walls > and chemically by the secreW P oured from their surface ; so that the nutritious parts of it are separated fiom those which may be rejected, and are reduced to a fluid form -That .^MF' ^1_ 142 OF ALIMENT, ITS INGESTION AND PREPARATION. the process of Digestion in Animals is really of no higher a character than this, and that it has nothing to do with ■ organising or vitalizing the materials submitted to it, appears alike from d, priori considerations, and from experiment. For the substances contained in the alimentary canal, and in contact with that reflexion of the external integument which constitutes its lining membrane, are really as much external to the living body, as if they were placed in contact with the skin ; we cannot regard them as introduced into it, until they have been absorbed; and up to that period, they hold precisely the same relation to the absorbent vessels, as the fluid diffused through the soil bears to the roots of Plants ramifying upon the surface of that Earth, which has been expressively said to be their ' common stomach. 5 All the experiments which have been performed upon artificial digestion, have precisely the same bearing; since it appears from them, that if the food be subjected to the action of the same solvent fluids, with the same assistance from heat and from mecha- nical movement, the result is the same out of the stomach as in it. 123. The particular articles which constitute the food of the different races of Animals, are as various as the races themselves. Some appear to draw their nutriment from the Inorganic world ; but this is not the case in reality. Thus the Spatangus and Arenicola fill their stomachs with sand, but really derive their nutriment from the minute animals which it contains. The Earth-worm and some kinds of Beetles are known to swallow earth ; but they only derive from it the particles of organic matter which it includes, and reject the rest.* 124. Some tribes in almost every division of this kingdom are main- tained solely by Yegetable food; and wherever Plants exist, we find Animals adapted to make use of the nutritious products which they furnish, and to restrain their luxuriance within due limits. Thus, the Dugong browses upon the submarine herbage of the tropics ; whilst the Hippopotamus roots up with his tusks the plants growing in the beds of the African rivers; the Giraffe is enabled by his enormous height to feed upon the tender shoots which are above the reach of ordinary quadrupeds ; the Rein-deer subsists during a large part of the year upon a lichen buried beneath the snow; and the Chamois finds a sufficient supply in the scanty vegetation of Alpine heights. Many species of Animals, especially among the Insect tribes, are restricted to particular Plants ; and, if these fail, the race may for a time disappear. But there is probably not a species of Plants, which does not furnish nutriment for one or more tribes of Insects, either in their larva state or their perfect condi- tion, by which it is prevented from multiplying to the exclusion of others. Thus, on the Oak not less than two hundred kinds of Caterpillars have been estimated to feed ; and the Nettle, which scarcely any beast will * Among the human race, some savage nations are in the habit of introducing large quantities of earthy matter with their food ; and this sometimes through ignorant preju- dice, but more frequently to give bulkiness to the aliment, so that the stomach may be distended, — as among the Kamschatdales, who mix saw-dust or earth with their train-oil. It has been until recently supposed that the siliceous earth, which has been employed in Lapland in times of scarcity^ mixed with flour and the bark of trees, merely answered this purpose; but recent microscopic examination has shown, that it consists of the exuvice of Infusoria, and contains a large portion of animal matter. If the latter be dis- sipated by incineration, the earth loses about 20 per cent, of its weight. - *. . J.-5-. • NATURE OF ALIMENTARY MATERIALS. 143 i touch, supports fifty different species of Insects, but for which check it would soon annihilate all the plants in its neighbourhood.— The habits and economy of the different races existing on the same plant, are as structure fruit will lew will eat nothing but the bark; while many derive their nourishment only from the woody substance of the trunk. — It is very curious to observe, that many plants injurious to Man afford wholesome nutriment to other animals j thus, Henbane, Nightshade, Water-hemlock, and other species of a highly poisonous character, are eaten greedily by different races of quadrupeds. Some cattle, again, "" upon which others feed with impunity. 125. Every class of the Animal kingdom has its Carnivorous tribes, also, adapted to restrain the too rapid increase of the vegetable-feeders' by which a scarcity of their food would soon be created, — or to remove from the earth the decomposing bodies, which might otherwise be a source of disease or annoyance. The necessity of this limitation becomes evident, if we consider the rapid multiplication which the prolific ten- dency of the Herbivorous races would speedily create, until checked b Y the famine that would necessarily result from their inordinate increase 1 hus, the myriads of Insects which find their subsistence on our forest trees if allowed to multiply without restraint, would soon destroy the hie that supports them, and must then aU perish together; but another tribe (that of the insectivorous Birds, as the Woodpecker), is adapted to derive its subsistence from them, and thus to keep within salutary bounds tne number of these voracious little beings. Sometimes, however, they increase to an enormous extent. Hart SSirjf iTS se ^ eral „ tlm ? s 7 almos * destroyed by the ravages of a single of 2 £\ • ?' the , JBostnchus typography, which is less than a quarter theLSf ? t 11 ? 1 ^ 6 eggS being de P° sited beneath the barl; and neil WtT f S Ch6d ' devourill g the alburnum and inner bark in their neighbourhood It was estimated that, in the year 1783, a million and LTLf Pl r- tr / es . w ?f e destroyed by this insect in the Hartz alone ; ana other _ forests m Germany were suffering at the same time. The wonder is increased when it is stated, that 80,000 larvse are sometimes tound on a single tree. Their multiplication is aided by their tenacity A ; Xt 1S found that ' even if the trees infested by these larvje be cut down, floated m water, kept for a length of time immersed either in water or snow, or even placed upon ice, the grubs remain alive and unhurt In the pupa state, however, they are more susceptible ; and vast numbers perish m this condition from the influence of unfavourable -«-—- which operate as the principal check to their multiplication curious instance of the nature of the checks and counter-checks bv which the 'balance of power' is maintained amongst the different r'ace^ too great, the IchneuIZZukmiJX^ ^^T^ ^ be ing its long tail in the opening fcl^^^tLSS •A very /. £ :■ * < 1 — -_• i-'_**' 144 OF ALIMENT, ITS INGESTION AND PREPARATION. insect, for its body is too large to enter. Thus it fixes upon the cater- pillar its minute egg, which when hatched destroys it.* 126. The Alimentary value of the various substances used as food by the several races of Animals, is not so different as, from the diversity of the sources whence it is drawn, we might be led to suppose. It depends, in the first place, upon the quantity of solid matter they respectively contain ; being of course the greater, as the solids form the larger pro- portion of the entire weight. Many a quantity of water, that the nutriment they afford is very slight in pro- portion to their bulk. — Next, it depends upon the proportion of digestible matter which the solid parts include ; for it is not every substance containing the requisite ingredients, that is capable of being reduced to a state which enables it to be absorbed. Thus, woody-fibre is composed of the same elements as starch-gum ; but it passes out of the intestinal canal of the higher animals unchanged, and therefore affords them no nutriment ; yet there are many tribes of Insects, which seem to draw their supply of nutriment exclusively from wood, and this even in its driest condition. So, again, the horny tissues of animals, though nearly allied to albumen in their composition, are completely destitute of nutri- tive value to Man and the higher animals, because not capable of being reduced by their digestive process ; though certain Insects appear capable of living exclusively upon them. — But when the watery and indigestible parts of the food are put out of consideration, and our attention is directed only to the soluble solids, we find most important relations in the chemical composition of the several alimentary materials, whether furnished by the Animal or the Vegetable kingdom, which render them more or less appropriate to the different purposes that have to be answered in the nutrition of the body. It is the remarkable attribute of Vegetables, that they are enabled to combine the elements furnished by the Inorganic world into two classes of compounds ; the ternary, con- sisting of oxygen, hydrogen, and carbon ; and the quaternary, which con- sist of these elements, with the addition of azote or nitrogen. These two classes are hence termed the non-azotized, and the azotized. 127. Now the azotized compounds which are formed by Plants, are essentially the same with those Albuminous substances which are fur- nished by the flesh and by the nutritious fluids of Animals ; and are equally adapted with the latter for the reparation of the waste of the muscular tissue, and for the general nutrition of the body. The quantity of these, however, which Plants yield, is usually small in pro- portion to that of the non-azotized ; being considerable only in the Corn- grains, and in the seeds of Leguminous plants, which the universal experience of ages has demonstrated to be the most nutritious of Vege- table substances. But, unless the food contain a sufficient proportion of these compounds, the body must be insufficiently nourished, and the strength must diminish, even though other elements of the food be in superabundance ; and consequently if the food be of a kind which con- * The Chapters on the 'Economy of Nutritive Matter' in Dr. Roget's * ' Bridgewater Treatise," and on the ' Equilibrium of Species' in Sir C. LyelPs u Principles of Geology," maybe referred-to for a more extended view of this interesting subject, than the limits of the present work permit. ^ w *i . e e 1 f e ir f NATURE OP ALIMENTARY MATERIALS. 14 ■ tains but a small proportion of albuminous matter, a very large amount oi it must be ingested, to afford the requisite supply of the essential ngrecUents. We see a provision for this requirement, in the capacity of fn 6 . mentai T canal of Vegetable-feeding animals; which is almost ^variably far greater than that of the Carnivorous members of the same groups.— There is another azotized compound, Gelatine, that is furnished J*+ • v^ t0 . wMch nothin S analogous exists in Plants ; this cannot f ustam lite by itself, and is not an essential article of food ; and there is n tact, much doubt whether it can be applied to the nutrition or repa- ration of any of the tissues of the body. There is ample evidence that * cannot be transformed into an albuminous compound, so as to be appli- abie to the nutrition of the muscular and other albuminous tissues. ^nd although the Fibrous substance which constitutes the animal basis ot bone, as well as the greater part of tendons, ligaments, skin, mucous and serous membranes, areolar tissue, &c. &c, has the same composition as Gelatine, and might therefore be presumed to be nourished by it when jt is employed as food, yet there is adequate evidence that even these tissues are generated in the living body at the expense of the albuminous constituents of the blood; and that, whenever gelatine is introduced into the circidating current, it is speedily decomposed and excreted serving only (like the non-azotized compounds) to assist in maintaining cne neat of the body. ft vJin!; J he non : az f iz 1 ed compounds supplied by Plants, exist under various forms ; of which the principal are starch, sugar, and oil. The FaJZ Vmer ma f be re g arded as belonging to one class, the Saccharine or it 7*nT° US; v h CaUSe y e kn ° W that starch ' and tLe substances allied to tisS It ° Se ' whi t . is the Palpal constituent of the vegetable that SvET ? con y. erted mto ^gar by simple chemical processes, and the An Ln T ^ nsformatlol ^ ake 1 s P^e readily both in the Vegetable and in ranked T * TT^ ° n the /^ hand > *« Oily matters are usually Stained +1 i i ^^ aWltar y ******** ; and it has been elaW^ }f: Under nc > circu ^tances, has the Animal the power of S is now 1 ^ ?t ter fr T ^^ ° r sacc ^ine compounds. But of I IT Wn t0 b6 / n unfounded imitation ; since the transformation oi a saccharine into a fatty compound takes place in the case of Bees, wnicn form wax when fed upon pure sugar ; and it may be effected also m the laboratory of the Chemist, butyric acid (the characteristic fattv acid of butter) being one of the products of the < lactic fermentation' of sugar, excited by Animal substances.— Thus, then, whether derived from Vegetable or from Animal bodies, the non-azotized substances available as food are essentially the same. The former kingdom supplies thZ chiefly m the saccharine or farinaceous form, the latter chifly in Z oleaginous; but a considerable quantity of oil is furnish pd hi + • Plants and there are Animals which have the ^T^It^™ lulose, like plants, and which store it nn \r, +W i *? enera tin_ Animal tissue to which the non-LL?d o T b ° dMa The 0nl y the appropriate .^^fff.^^ apparently serve as eel- the appropriate pabulum, is the Adir>o J 2*1? IP" believe, however, that oleaginous ™hT / a% = there 1S reason to in the incipient stages of S ST^i" Tf *"**** ** less essential to tfJ W Q+ ™ * U „ tl0 ?> and tllat lts presence is not less essential to the formation of cpIiT +/ Ana ™™ its presence is matter whfoh fn™* «. 2 S °* - Cells ' tlla \l s th f, of the albuminous 11 such be the case, it is not matter which forms their chief component. L ■ 146 OF ALIMENT, ITS INGESTION AND PREPARATION. surprising that oil in some form, or substances capable of conversion into it, should be such universal constituents of the food of Animals. serving pounds have a most important use in warm-blooded Animals ; that of supporting the respiratory process, and thus maintaining the tempera- ture of the body. In the compounds of the Saccharine group (in which Starch is included), the amount of oxygen is no more than sufficient to form water with the hydrogen of the substance; so that the carbon is free to combine with the oxygen introduced by the lungs, and thus becomes a source of calorifying power. In the Oily matters employed as food, the proportion of oxygen is far smaller ; so that they contain a large quantity of surplus hydrogen, as well as of carbon, ready to be burned-off in the system, and thus to supply the heat required. extraordinary of oleaginous substances to impart heat to the system by the combustive process, is indicated by the experience of the Human inhabitants of frigid zones, who feed upon whales, seals, and other animals loaded with fe.f. arid who rlp.vonr this fat with avidity, as if instinctively guided to T its use. It is through the enormous quantity of this substance taken-in bj them, that they are enabled to pass a large part of the year in a tempe- rature below that of our coldest winter, spending a great portion of their time in the open air ; as well as to sustain the extremes of cold, to which they are occasionally subjected. And in consequence of its being more slowly introduced into the system than most other substances, a larger quantity may be ingested at one time, without palling the appetite; whilst its bland and non-irritating character favours its being retained until it is all absorbed. In this manner, the Esquimaux and Green- landers are enabled to consume 20 or 30 pounds of blubber at a meal ; and, when thus supplied, can pass several days without food. — On the other hand, among the inhabitants of warm climates, there is compara- tively little disposition to the use of oily matter as food ; and the quantity of it contained in most articles of their diet is comparatively small. 130. The greatest economy in the use of Aliment is therefore exer- cised, when the diet contains a sufficient proportion of albuminous sub- stances to repair the i waste' of the albuminous and gelatinous tissues ; and a sufficient amount of non-azotized compounds, to develope (with the aid of other processes) the requisite amount of heat by combination with Now in the Milk, which is the sole nutriment of young Mam- iring the period immediately succeeding their birth, we usually find an admixture of albuminous, saccharine, and oleaginous substances ; which seems to indicate the intention of the Creator, that all these should be employed as components of the ordinary diet. The Caseine, or cheesy matter, is an albuminous compound; the Butyrine of butter is but a slight modification of the ordinary fats; and the Sugar differs from that in common use, only by its larger proportion of water. The relative amount of these ingredients in the milk of different animals, is subj ect, as we shall hereafter see, to considerable variation ; but they are con- stantly present in the milk of the Herbivorous Mammalia, and of those which, like Man, subsist upon a mixed diet. It has been recently found, however, that the milk of the purely Carnivorous animals is destitute of Sugar, consisting, like their food, of albuminous compounds and fatty matter only ; though even their milk is found to contain sugar, when oxygen D t e NATURE OF ALIMENTARY MATERIALS. 147 saccharine or farinaceous compounds have formed part of their diet. — No tact in Dietetics is better established, than the impossibility of long sus- taining health, or even life, upon any single alimentary principle. Neither pure albumen or fibrine, gelatine or gum, sugar or starch, oil or fat, taken alone for any length of time, can serve for the due nutrition of the body. J-lus is partly due, so far as the non-azotized compounds are concerned, to their incapability of supplying the waste of the albuminous tissues. £>ut this reason does not apply to the albuminous compounds ; which can not only serve for the reparation of the body, but can also afford the carbon and hydrogen requisite for the sustenance of its temperature. J-ne real cause is to be found (partly at least) in the fact, that the con- tinued use of single alimentary substances excites such a feeling of uisgust, that the animals experimented-on seem at last to prefer the endurance of starvation, to the ingestion of them. Consequently it is quite impossible to ascertain, by such experiments, the nutritive power of the different alimentary principles ; no animal being capable of sus- taining life upon less than two of them. The same disgust is expe- rienced by Man, when too long confined to any article of diet which is y ery simple in its composition ; and a craving for change is then expe- rienced, which the strongest will is scarcely able to resist. The natural combinations in which the alimentary substances present themselves, appear to be those which are best adapted for the healthful nutrition of the animal body. " 1 +L ' The 0r S anic Compounds, which have been enumerated as supply- ing the various wants of the system, would be totally useless without the omixture of certain Inorganic substances, which also form a constituent part ot the bodily frame, and which are constantly being voided in the excretions, especially in the urine. m the system. ~~ These substances have various uses , . - Thus common Salt (chloride of sodium) appears to afford, oy its decomposition, the hydrochloric acid which is concerned in the di^es- ive process, and the soda which is an important constituent of the bile its presence in the serum of the blood, also, and in the various animai nuias which are derived from this, aids in keeping in solution the organic constituents of these fluids, and in preventing their decomposition.— The iswrbonates and Basic Phosphates of Potass and Soda are most important con- stituents of the blood; and potass is also an essential component of muscular tissue. — Phosphorus seems to be chiefly requisite as one of the materials of nervous y ' ~ "J ~"-./a~" «""«. "iUUCU WILD lime, forms the bone-earth by which bone is consolidated.— Sulphur exists m small quantities in several animal tissues ; but its part appears to be by no means so important as that performed by phosphorus —Iron is an essential constituent of haematine; and is consequently required for th« production of the red corpuscles of the blood in Vertebrated animals Lime is required for the consolidation of the bones of Vertehrafo 7^V" the shells and other hard parts that form theX^rf^lS^ brata: and it exists m the animal iw^ : i- j- - ^ -^verte- carbonic or with phosphorraci^ [ Th, rlV combination either with a***- a 4. i^>pnuiic acia. liie Carbonate would seem princiiialW tZ T"i T™ 1 ™ 1 ^™ "**'> and we find * Predominatinrr e xtt y T 1 + ^ Ta mme i ^ T? redie ^ m those non-vascular tissue s of it IZT ^ amma > 7 hiGh giYe SU PP° rt and P rotec ti°n to their soft parts. The amount of production of these tissues depends in. P ! part upon the supply of carbonate of lime which the animals reSve l2 148 OF ALIMENT^ ITS INGESTION AND PREPARATION. • The large amount of carbonate of lime Thus the Mollusca which inhabit the sea, find in its waters the propor- tion of that substance which they require; but those which dwell in streams and fresh- water lakes that contain but a small quantity of lime, form very thin shells; whilst the very same species inhabiting lakes, which, from peculiar local causes, contain a large impregnation of calca- reous matter, form shells of remarkable thickness. The Crustacea, which periodically throw off their calcareous envelope, are enabled to renew it with rapidity, by the appropriation of a store of material previously laid- up in the coats of the stomach, which is required by the laying Hen, is derived from chalk, mortar, or other substances containing it, which she is impelled by her instinct to eat; and if the supply of these be withheld, the eggs which she deposits are soft on their exterior, having the fibrous element of the shell uncon- solidated by any intervening deposit of calcareous particles. 132. These substances are contained, more or less abundantly, in most of the articles generally used as food ; and where they are deficient, the animal suffers in consequence, if they be not supplied in any other way. — Common Salt exists, in no inconsiderable amount, in the flesh and fluids of animals, in milk, and in the substance of the egg; it is not so abundant, however, in Plants ; and the deficiency is usually supplied to herbivorous animals from extraneous sources. Thus, salt is purposely mingled with the food of domesticated animals ; and in most parts of the world inhabited by wild cattle, there are spots (such as the " buffalo-licks" of North America) where it exists in the soil, and to which they resort to obtain it. — The Alkaline Bases are supplied both by Vegetable and by Animal food ; what is drawn from the former, however, is usually com- bined with some organic acid, which is # decomposed within the animal body; where also is generated, by the oxidation of phosphorus, the phos- phoric acid that is found in combination with them. — Phosphorus exists also, in combination with albuminous compounds, in all animal substances composed of these ; and in the state of phosphate, combined with lime, magnesia, and soda, in many substances, both vegetable and animal, ordinarily used as food. — Sulphur is found in union with albuminous compounds, in flesh, eggs, and milk; also in several vegetable sub- stances; and, in the form of sulphate of lime, in most of the river and spring water used as drink. — Iron, also, is very generally dif- fused, in small quantity, through the tissues of plants ; but it exists in much larger proportion in the flesh, and more particularly in the blood, of animals; and it appears from comparative analyses of the blood of Carnivorous and Herbivorous Mammals, that the proportion of red cor- puscles, and consequently of iron, is greatest in the former ; which cir- cumstance seems partly attributable to the nature of their diet. — Lime is one of the most universally diffused of all mineral bodies; for there are very few Animal or Vegetable substances, in which it does not exist. The principal forms in which it is an element of Animal nutrition, are the carbonate and phosphate. Both these are found in the ashes of the grasses, and of other plants used as food ; the phosphate of lime being particularly abundant in the corn-grains. Phosphate of lime unites readily with albumen, caseine, &c. ; and a large quantity of it is contained in the milk of the Mammal, and in the egg of the Bird. 133. The dependence of Animal life upon a constant supply of aliment, NATURE OF ALIMENTARY MATERIALS. 149 *s more close in some eases than in others. As a general rule it is the most immediate, when the vital processes, particularly those of Nutrition, a **e being most actively performed. Thus, we find that young animals a ^e never able to bear the deprivation of food to the same extent with older ones of the same species ; and that the warm-blooded Yertebrata — are usually less capable of abstinence than Mammal Reptiles and Fishes. Even of the first of these classes, however, many species pass several months without eating, during the state of hyberna- tion ; whilst among many cold-blooded animals, the period of abstinence from food may be indefinitely prolonged, under the influence of those a gencies which keep them in a state of complete torpidity or ' dormant vitality'. (See General Physiology). — When we carry our enquiries further, however, it becomes difficult to give any general explanation of the Varieties which we meet- with in this respect, among the different species °t animals. Thus, it has been observed by Flourens and Duges,"* that the Mole perishes, when in a state of confinement, if not fed every day, °r even more than once a day ; whilst the Dog has lived without food for ^eeks. Wild respect. It is in Reptiles, that the power of abstinence appears to exist ^ le greatest extent among Vertebrata. Putting aside those cases in which the natural period of torpidity has been artificially extended, we ■ftd numerous instances in which these beings have performed all the unctions of life for many months together, without the ingestion of food ; >^ rt0 *f eS ' Wizards, Serpents, and Batrachians all seeming to agree in this It is to be borne in mind, however, that a large supply of food x " iiCt L ue ntly ingested at once by these animals; and, that, owing to the slowness of ^ their digestive process, the introduction of the aliment into e system is protracted over a very long period, — as is seen, for example, m the case of the Boa constrictor, which occupies a month in the diges- ion oi a single meal. Little is known regarding the powers of absti- nence possessed by Fish ; but it has been stated that some of this class, & uch as the Perch, naturally take food but once a fortnight, t— It is perhaps among the Insect tribes, that we find the power of sustaining a deprivation of aliment the most remarkably evidenced. The Scorpion lias been known to endure an abstinence of three months, the Spider of twelve months, and the Scarabceus beetle of three years, without inconve- nience or loss of activity ; the Melasoma, also one of the beetle tribe, has lived for seven months pinned-down to a board. We notice in the class of Insects a very striking illustration of the general fact already stated respecting the difference between the old and the young animal of the same species. The Larva is not only extremely voracious, but is usuallv incapable of sustaining a long abstinence; whilst in many tribes the Imago never eats but dies as soon as its share in the propagation of the race is accomplished. -From what is known regarding the power of -* inence m the Mollusca, it may ' '•' Physiologie Comparee," torn. ii. p. 288. are it is requisite that the water they inhabit ^Wf J + i T a mj i ° ther wa ?> quantity of organic matter which H holds ?n J\- ^ 7 Z^l'' and the minute aliment. ° ld& m solutlon > 1S probably the source of their _ ^m 150 OF ALIMENT, ITS INGESTION AND PREPARATION. I not capable of maintaining their activity if not frequently supplied with food, in this respect corresponding with the larvse of Insects ; but that, when reduced to a state of torpidity, whether by cold or by the depriva- tion of food, they may sustain life without any aliment for a very pro- tracted period. 134. Some have attempted to show that Herbivorous animals are more dependent upon a constant supply of food, than Carnivorous species ; and that domesticated animals less easily sustain a deprivation of it, than wild ones. But these statements, though generally true, are found to be want- ing in accuracy when applied to individual cases. It would probably be more correct to state, that, in proportion to the facility with which each species usually obtains its food, will be the directness of its dependence upon this, and its inability to sustain a protracted abstinence. In accord- ance with this principle, we observe that, though vegetable-feeders have in general their food within reach, and are very dependent upon its con- stant supply, some, as the Camel, are enabled to sustain abstinence better than many Carnivorous species ; and that some carnivorous animals, being enabled from the nature of their food to obtain it with little difficulty, are comparatively unable to bear the want of it. On the same principle it is evident that domestication may induce a change in the character of the animal, in this respect, as in others, by causing it to become accustomed to frequent jpr constant supplies of food. — A like adaptation may be found Those which feed upon vegetables or upon dead animal matter, speedily die out of the reach of their aliment ; whilst those that lie in wait for living prey, the supply of which is uncertain, are able to endure a protracted abstinence, even to the extent of ten weeks, without injury.** 135. The general facts which appear to have been substantiated, in regard to the mode in which the Animal body is sustained by Food, may be thus summed up : I. The waste of the tissues of which Albmnen or Gelatine is the basis, must be supplied by Albuminous compounds, whether these be derived from the Animal or from the Vegetable kingdom ; the amount of this < waste,' and consequently the demand for albuminous aliment, depends essentially upon the degree of vital activity which has been put forth, and especially upon the exercise of the nervo-muscular apparatus ; and there- fore, cceteris paribus, it is greater in cold-blooded animals in proportion to the elevation of the external temperature. ii. The materials of the Adipose tissue, and the oleaginous particles which seem requisite in the formative operations of the system generally, are derived in the Carnivorous races, from the fatty substances which the bodies of their victims may contain j whilst the Herbivorous not only find them in the oleaginous state in their food, but have the power of producing them by the conversion of farinaceous and saccharine matters. among the Larvse of Insects. in. The foregoing statements are applicable to all tribes of Animals, -A A \A*-x^Ar\A oo iirnl oci £-«•»*%.«*«*» 1~1 I- J .9 - 1 _i_ _ • _l ii warm-blooded ;' we have now to consider the —In the Carnivorous tribes the ' waste ' of the 1 cold-blooded ' as well as special case of the latter! tissues is so great, ill consequence of the restless activity which is habitual to them, that it appears to furnish a large proportion of the combustible * Lacordaire, "Entoinologie," torn. ii. p. 152. STOMACHAL CAVITIES IN PLANTS. 151 material required for the maintenance of their proper temperature. The remainder is made-up by the fat of the animals upon which they feed ; &&d it is to be observed that the amount of this is much greater in the bodies of animals inhabiting the colder regions of the globe, than in the inhabitants of tropical climates. — In the Herbivorous tribes, the case is different. They are, for the most part, much less active ; and the i waste ' of their tissues consequently takes place in a less rapid manner, and is far from supplying an adequate amount of combustible material, especially in cold climates. Their heat is in great part sustained by the combustion of the saccharine and oleaginous elements of their food, which are appro- priated to this purpose without having ever formed part of the living tissues ; and the demand for these will be larger in proportion to the de- pression of the external temperature, a greater generation of caloric being then required to keep-up the heat of the body to its proper standard. IV. Hence 'cold-blooded' animals can usually sustain the privation of food longer than warm-blooded ; and this more especially when they are kept cool, so that they are made to live slowly; and death, when at last it does ensue, is consequent upon the general deficiency of nutrition. On the other hand, < warm-blooded ' animals, whose temperature is uniformly high, must always live fast; and deprivation of food is fatal to them, not only by preventing the due renovation of their tissues, but also by de- stroying their power of sustaining their heat. The duration of life under these circumstances depends upon the amount of fat previously stored-up m the body, and upon the retardation of its expenditure by external warmth, or by the enclosure of the body in non-conducting substances ; and there is evidence that, if this be duly provided for, and all unneces- sary waste by nervo-muscular activity be prevented, the life even of a warm-blooded animal may sometimes be prolonged for many weeks with- out food. I 3. Ingestion and Preparation of Aliment in Plants. 136. Although, as already explained, the Vegetable world as a whole is supported by the introduction of the alimentary materials derived from the Earth and Air into the organism, without any preliminary alteration, yet there are particular cases in which adaptations of structure are met with, that appear to be subservient to the reception and preparation of nutritive materials ; and some of these it would not be easy to exclude from any definition we might frame of a ' stomach.' Concavities in dif- ferent parts of the surface, fitted for the collection of the moisture caught from rain or condensed from dew, may frequently be observed ; and these vary in the complexity of their structure, from the simple depressions in the leaves of the Tillandsia (wild pine of the tropics) or of the Dipsacus (teasel), to the extraordinary ascidia of the < Pitcher-plants.' The exact method in which the fluid thus obtained is applied to the nutrition of the plant, is not always evident.* Sometimes the channelled leaves seem to convey it to the roots, by which it is absorbed in the usual manner. In * It is difficult to ascertain how much of the fluid which the pitcher of the Nepenthes or Chinese pitcher-plant contains, is collected from the atmosphere by the downy hairs that line its interior, and how much is secreted by the plant itself ; and it is certain that the young pitcher contains fluid secreted from an organ within it, before its lid first opens I 152 OF ALIMENT, ITS INGESTION AND PREPARATION. the Dischidia (Fig. 89), on the other hand, the fluid collected by the pitchers seems destined to be more Fig. 89. directly appropriated by the plant, through an absorbing apparatus provided for that purpose. This curious plant grows by a long creeping stalk, which is bare of leaves until near its summit ; and as, in a dry tropical atmosphere, the buds at the top would have great difficulty in obtaining mois- ture through the stem, a sufficient supply is provided by the pitchers, which store up the fluid collected from the occasional rains. "The cavity of the bag," says Dr. Wal- lich* is narrow, and always ■ Pitchers of" Dischidia Hafflesiana, with tufts of root- lets prolonged from neighbouring branches ; a, pitcher cut open, to show the ramification of the rootlets in its interior. contains a dense tuft of radicles, which are produced from the near- est part of the branch, or even from the stalk on which the bag is suspended, and which enter through the inlet by one or two common bundles. Ihe bags generally contain a great quantity of small and harmless black ants, most of which find a watery grave in the turbid fluid which frequently half-fills the cavity, and which seems to be entirely derived from without." Thus it would seem as if the failure of the ordinary means of support in this curious Plant, has been compensated by the addition of an organ, which like the stomach of Animals, serves as a receptacle for the supplies it may occasionally obtain.— According to Mr. Burnett, t in the pitcher of the Samacmw,, a process still more like that of animal digestion goes on j for it appears that the fluid it contains is very attractive to insects, which, having reached its surface, are prevented from returning by the direction of the long bristles that line the cavity. The bodies of those which are drowned seem, in decaying, to afford a supply of nutri- ment that is favourable to the growth of the plant ; like the similar process known Although health of which a supply of animal food appears to be essential, such instances as these may seem to contradict the general statement that Plants derive the materials of their nutrition from the inorganic world yet they probably do so more in appearance than in reality. In all cases where previously organised matter influences their growth, it seems to do so only whilst in a decomposing state, during which it is separated into its ultimate elements or into very simple combinations of them. In Animal digestion, on the contrary, the proximate principles contained in the food appear to be immediately subservient to the formation of others of a higher order; and whatever tendency to disunion their elements might have previously manifested, this is immediately checked by the antiseptic qualities of the gastric fluid. * "Plantse Asiatics Rariores," vol. ii. p. 35. t " Brande's Quarterly Journal of Science," vol.* vi. ' AGASTRIC ANIMALS. 153 of A liment , 137. In considering the various organs which Animals possess for the jwgestion and digestion of their food, it is right to take notice, in the first instance, of those cases in which there appears to be an absence of that provision for its reception, which is on the whole so characteristic ■w the beings included within the limits of this kingdom. — There are several examples, even amongst animals of high organisation, in which, uurmg the last stage of their evolution, there is an entire absence of any power of receiving nourishment into the system. These are principally fliet-with in the class of Insects ; among which there are many that take uo food in their perfect or Imago state, the duration of this being very short, and serving merely for the performance of the reproductive act. in some of these cases, the mouth appears to be actually closed, although he digestive apparatus remains, but in an atrophied condition: in a u, however, the appropriation of food has been actively performed during a previous stage of the animal's existence. But an animal has been recently discovered, which takes-in no nutriment, from the time of "s quitting the egg, until its death, and which does not possess either oral orifice or digestive cavity ; this is the male of the curious Notom- mata^ described by Mr. Dalrymple,* which, being entirely incapable of deriving support from the nutriment upon which the female subsists, has no apparent means of increase or maintenance, after it has exhausted the store prepared for it in the egg j and it is probable that the duration of s uie is extremely limited, and that it ceases to exist soon after it has pertormed its part in the reproductive function. — Such instances are t really so exceptional as they at first sight appear ; we have now incTa^ • th0Se ° aSeS ' in Whi ° h the S rowtn and development of beings a tvh ■ ? i the Animal kin g<*om take place at the expense of aliment J£w P , by tnemselves i an d in which there is, nevertheless, no semblance of a digestive cavity for its reception. «ii + A A9astric Animals .—The only Animals which can be properly baia to be nourished by imbibition solely, without some previous opera- won ot a digestive character, are the Cestoid Entozoa (Tapeworm, &c. ) ; wnicn, according to the most recent and accurate researches, are not only unpossessed of a mouth, but also of anything equivalent to a gastric cavity or intestinal tube, t These creatures are supported by the j uices ot the animals which they infest ; and as the softness of their bodies fits them to imbibe these through their whole external surface, no more special organization appears to be necessary. The transition from these to the unicellular Protozoa is made by the curious Gregarlna + the simplest Entozoon known j being a single cell, usually more or less "ovate in form, sometimes considerably elongated, with a beak or proboscis (often ^t ttZ- ?7% GirG 4 ■ T Of ^ ooklets ) Projecting from one extremity 139. Umcellular Ammcds.-The condition of those simple Protozoa in which every cell is an independent < zooid,' in regard to their mode of receiving and appropriating food is so ™™Uo + • IJaoae . ot notice. Although they are destitute rf ? **J° rGqmre Special 6 uuey * mm E TF H v. // ^SK it n in, u***/i -. *V ft > wmmm i Polygamic Animates according to Ehrenberg :-a, individual monad of Fofe oa; global showing a, the supposed month ; 6, b gastric vesicles ; ., t, t, contractile vesicles f W cor^ of communication j-b, individual of Vorhcella eitrina; a, the stalk; s, contractile vesicles ® «, intestinal tube ;-c, external aspect of JSnchelis pupa in the act of taking food, showing a the mouth, and b the anal onflce ;-D, supposed digestive apparatus of the same animalcule • e,f, similar views of Leucophrys patula; g, Chilodon oucullulus ; showing a the dental am>a- ratus; h, i, Paramecium aurelia, showing a the mouth, 5 the anus, « contoactOe vesicles. PP 141. The transition from the Unicellular animals, in which, however great the aggregation, every cell repeats the rest, to those higher organisms in which the parts of the aggregate mass are completely differentiated from each other, is effected (as already pointed out, § 35) through the group of Sponges ; and in no part of the structure is this transition more evident, than it is m the provision for the reception of food. For a mass of fcponge may be regarded, on the one hand, as an aggregation of Amceba- isIbKo^fi^VM^ 11 "" ^^ Hi f->'' Vo1 - m -> PP. 100 and 170. Theauthor serrations ^ ^ essenti al particulars, from his own ob- Ill POLYSTOME ANIMALS. 157 like cells, each one living for and by itself, but all being held together by a common integument, and supported by a framework which all participate m. forming. Or, on the other, it may be looked-at as a whole, and the attention chiefly fixed upon the system of canals which traverses the body, and upon the circulation of fluid that goes-on within these, by the agency of the cilia with which they are lined. These canals must be con- sidered as altogether forming an alimentary cavity, which is so extended through the mass, as to supply to every part of it the materials of its growth ; whilst the manner in which the several Sponge-cells draw their aliment from the circulating current, finds its exact parallel in the manner in which the individual cells that enter into the composition of the most highly-organised fabric, nourish themselves by imbibition from the fluid that bathes their exterior (§ 166). 142. Polystome Animals. — In all animals whose entire bodies are repeated by the process of gemmation (§ 34), but whose cavities remain in connection with one digestive another, the entrance to each division of the stomach is properly to be accounted a distinct mouth ; and thus the entire compo- site fabric may be said to be ' polystome' or many-mouthed. This is the case amongst all the s composite' Zoophytes, whether Hydra- form, Actiniform, or Alcyonian (§§ 151, ^°~>) : and by that early form of the spongoid basis of the latter (Fig. 91), in which the Polypes are not yet developed at the extremi- ties of the canals, we are carried back to the true Sponges, in which the canals never be- come furnished with polypes. It is equally the case among the Composite Acalephce, Jhose strange forms, it is now satisfactorily determined, are the result of a process of gemmation, which multiplies the entrances to the complex digestive cavity, that ex- tends continuously (as in the Hydroid Zoophytes) through all the parts thus pro- duced.* — There is no known instance of a multiple mouth in any other animals than such as thus acquire a composite structure ; for the Rhizostoma (Fig. 92} does not Fig. 91. constitute a real exception. In this animal, Portion of a young branch of the polypidom of Alcyordum stellatmn divided longitudinally to show the spongiform character of its structure • a, incipient state of young polype. ' Medusae, we find in ^i«^ ./ * This very important modification of the views fornwlv an**,*^^ Physograde, Cirrhigrade, and Diphyd Meduss This ^^X^t ^^ the of Mr. T. H. Huxley communicated to the Cl St! bro W about by the researches of Dr. Rudolf Leuckart, publish^ his Monograph of the SiphonophoraTin Bffi i? tt I B Zei * sclmft > 1851 > a *<* in those of Prof Kolliker "Die S^J^J^J^^ Vnt ™™ h ™Z™> " 1853 ; and by It is much to be regretted ffiH&fe 1853 -~ the Physophorid* and the Dij^T?£ S^W* *' ^t* Mmoir °n prived him of the credit to which heTs enLld »I S *T ^i sWd We de- idea of the nature of these perpleKfon^^ ^ enunmtor * the correct <" *l I ^■^ * le58 OF ALIMENT, ITS INGESTION AND PREPARATION. il | an open mouth in the centre of its eight tentacula, the tentacula them- Fig. 92. selves excavated by canals, which communicate with the single cavity of the Rhizostoma injected with coloured fluid, to show the central digestive cavity, and the canals branching-off from it to ramify in the arms and in the disk. stomach, and which ramify and subdivide, until their finest branches open on the surface by pores that are too small to allow any save very minute Animalcules to enter. But, as Eisenhardt has shown/* the Rhizostoma differs from other animals only in this ; that its mouth (which is really single) does not open directly outwards, but is provided with a num- ber of tubular appendages, which extend themselves into the foliaceous expan- sions of the arms, so as to augment, as much as pos- sible, the number of points through which the alimen- tary particles required by this animal may be absorbed. — Here, then, we have a sort of transitional form between the composite ' polystome' Acalephs, and those ' monostome' tribes which correspond with all higher animals in possessing but a single external entrance to the digestive cavity. The relative size and form of the single oral 143. Oral Apparatus.— orifice that is common to all other animals, differ very greatly, according to the nature of the food upon which the species is destined to live. The Invertebrate division contains many groups, that are supported by suction of the juices of higher animals or of plants; and in them we usually find the oral orifice narrowed and sometimes immensely prolonged. As such a means of obtaining food usually involves the necessity of locomotive powers, whereby the animal may go in search of it, we find, as might be expected, that it is most common in the Articulated sub-kingdom ; all the principal classes of which, — Entozoa, Annelida, Insects, Crustaceans, and Spiders, — contain groups of greater or less extent, which are especially adapted for obtaining their food by suction. The Pycnogonidce (Fig. 105) present a characteristic example of the most simple form of suctorial mouth, in which the aperture is merely narrowed and somewhat pro- longed ;t but the most remarkable modifications for this purpose are to * "Nova Acta Leopold," torn x. p. 392. t From the situations in which these creatures are commonly found, the Author would infer that they draw their nourishment from the mucus that covers the surface of sea- weeds. ORAL APPARATUS. 159 •erous -Dipterous orders. Of the first of these groups, a considerable proportion feed upon the juices of flowers, which the large expanse of their wings prevents them from entering ; and their long haustellium, which is coiled U P beneath the head when not in use, can be so extended as to suck up the honey from the bottom of a deep blossom, while the insect rests upon if a ^,j._.. _.!.._ Q£ ^e latter, however, a considerable proportion feed its outer edge. upon the juices of animals; but there are some that have recourse to the honeyed exudations of flowers ; and among these the JSfemestrina longi- Tostris (a Dipterous insect of the Cape of Good Hope) deserves especial niention, the length of its proboscis being about 3 inches, whilst that of its body is only about 8 lines; so that it is enabled to feed upon the juices of a flower whose tubular corolla equals its trunk in length. Many animals whose mouths are adapted for suction, possess some means of attaching themselves to the spots in which they can best obtain the nutriment they require ; thus we find the Cystic and Cestoid Entozoa very commonly provided with a set of hooklets on their heads ; the Trematoda usually have the mouth surrounded by a circular sucker (Fig. 100, a); the Suctorial A nnelida have not merely a sucker for attachment, but an incising apparatus for making incisions into the blood-vessels of the animals whose juices they suck ; the Suctorial Crustacea usually have a sucker formed by the peculiar development of one of the pairs of members, ymlst the mouth is elongated into a proboscis armed with penetrating instruments ; and the Suctorial Insects, which seldom attach themselves permanently in any one situation, but make a fresh puncture whenever tneir appetite incites them to feed, are for the most part destitute of any pecial organs of adhesion, but have a most elaborate set of instruments or seising the skin of the animals they attack— The Cyclostome Fishes \a?\ \ • ' A ) are the onl y Vertebrated animals, which present any similar adaptation to a suctorial mode of nutrition. fi, ^ tu' ^ rhen the food consists of solid matters, which is the case in b y iar the larger proportion of the Animal kingdom, we find the entrance to tne digestive cavity of much greater proportionate size ; and it is jeidom prolonged forwards, but far more frequently has somewhat of a runnel shape, so that the aliment may be more readily drawn within it. J- he means by which the introduction of food is accomplished, are ex- tremely various. The simplest plan is one which carries us back to the type of the Infusoria, although it presents itself in a much more elaborate form. Among many aquatic animals, whose food consists of minute particles diffused through the water, we find that the action of cilia situated around and within the entrance of the alimentary canal or nnrm' organs m immediate connection with it, whereby a current of water may LtrThe^ r^kabi: ttmlsTfTnt^ ?* f^ k "^ agency^re Wished by the ^^SS^ unfitted for prehension, and effect the pro- La- introduction of alimentary matter solelv b v 2 ^ ensl0n ' ai f f ec duce ; and the Tunicail (kT 42 7?^j ^c^* **"* mellibranchiata (Fig. 43) are Yll ?' ?.™ chw P° da (§ 138), and essentially the sani AmoT, R ad S ■ *f **""* * a maimei> quently had to this P C whifh t t ^ ""J™ 6 S6ems less fre " tW forms of the d^^^S ^2^1t t M 160 OF ALIMENT, ITS INGESTION AND PREPARATION. the requisite current could not be effectually maintained through such j but among the Giliograde A calephm (Fig. 101), which possess an anal orifice, the ciliary action appears to be the means of introducing aliment, as well as of propelling the body through the ocean. In the Articulated classes, too, this ciliary action is seldom the means of introducing aliment into the stomach ; most of them feeding upon solid bodies of comparatively large dimensions, and being enabled by their locomotive powers to go in search of their food. But among those which resemble in their habits of Molluscous currents by the cilia clothing the respiratory tufts which are seated on their heads, that the Serpulce and their allies (Fig. 144) draw in their food- and in Rotifi subservient ration and to the ingestion of aliment. It is probable that the Amphioxus (Fig. 127) is nourished in a similar manner; but that remarkable animal affords the only known example among the Vertebrata, of the employ- ment of ciliary action for this purpose. It is curious to observe, however, in the Greenland Whale, a mode of ingestion which is essentially the same, though accomplished by a different instrumentality j for this huge animal gulps enormous volumes of water into its capacious mouth, and then ej ects these through its blow-holes, straining out, through its whale- bone-sieve, the small Medusse, Pteropods, Crustacea, Fishes, or other free- swimmmg marine animals, which the water may contain ; it beino- such alone that it is capable of swallowing. — The foregoing plan is one that seems intermediate, in regard to the kind of nutriment for which it is adapted, between the suctorial method, which is fitted only for the intro- duction of liquid aliment, and the one we have next to consider, which is appropriate to the ingestion of solid masses. 145 The organs with which the Digestive Apparatus is provided, for the introduction of solid food into its cavity, vary greatly in complexity m the different tribes of animals which derive their subsistence from aliment of this kind. Thus we shall find that in the lowest and simplest types these organs are in immediate connection with the stomach; but that they are separated from it in the higher by the interposition of an cral apparatus, with prehensile appendages for laying hold of the food which after passing through the buccal cavity, is drawn downwards into the stomach ; and that in the highest of all, the introduction of food into the mouth is itself aided by acces- sory organs, which do not form part of its own organization. Of the first of these types, we have a character- istic example in the Hydra (Fig. 34), the entrance to whose stomach is surrounded by ' tentacula,' which may be considered as representing the ., „ f ^ -KT , , pharyngeal and oesophageal muscles Thawmanttas pilosella, one of the Naked- l.p i, • i • i mi . -, eyedMedusa; :— aa.oraltentacula; 6, stomach; OI Ulgner animals. lne tentacula of c, gastro-vascuiar canals having the ovaries the Actinia (Fig. 35) correspond with a d, on either side, and terminating in the , . V & > l vuu " ± vxl marginal canal e e. tnese m character and in mode of Fig. 93. ORAL APPARATUS. 161 action; and it maybe said that the same type prevails through the whole class of Polypifera. The Medusce and their allies are formed upon a plan which ttiust be considered as essentially similar, the four tentacula (Fig. 93, a a) being there, also, placed around the entrance to the stomach; and it is by the adhesion of the edges of these tentacula, that the 'proboscis' is formed in such Wheth °f the Pulmograde Acalephse as possess it. It is interesting to observe in the class of Echinodermata, a gradual transition towards a more elevated type. Thus in the Asterias (Fig. 37), which has no oral apparatus, we find the food brought to the stomach, not by tentacula, but by the flexion of the lobes of the body, commonly known as their i arms. 5 " Starfishes," says Prof E. Forbes,* " are not unfrequently found feeding on shell-fish ; in such cases they enfold their prey within their arms, and seem to suck it out of its shell ^ith their mouths pouting out the lobes of the stomach. They can project the central parts of their stomachs in the manner of a pi the true 'arms' of the Crinoidea (Figs. 9, 38) and of OpMurida (Fig. 8), serve to bring food to the stomach, is not certainly known ; but their Position in the former of these orders, at least, would lead to the infer- ence that they are subservient to this purpose. In the Echinida and Holothurida, however, we find that the entrance to the alimentary canal is no longer the entrance to the stomach ; but that the former becomes a true c mouth,' being furnished with a prehensile apparatus of its own, a frd being separated from the stomach by the intervention of the oeso- phagus. The mouth is Provided, in the typical Echinida, with a dental apparatus (Fig. 95, a), which is capable of being proj ected beyond the oral orifice ; whilst in the Holothurida (Fig. 94), it is surrounded by prehen- sile tentacula, which are here to be regarded as labial, being extensions of the lips, rather than as a divided pharynx. — Turning to the Articu- Fig. 94. «y^ ftiA fa -«*■ -J^ggS JTolothuria phantapus. lated series, we find that in a considerable proportion of the Annelida, the mouth is furnished with powerful jaws, sometimes to the number of three pairs, opening laterally; or with a proboscis which is capable of being everted and which then displays an armature of teeth on its exterior (Fig. 53 b c) In the Mandibulate Insects, and in the greater part of the Crustacean class, we find the jaws highly developed, and taking the place of anv other kind of apparatus ; the former group always possessing two pairs of these organs, which open laterally, one above and the other below the oral orifice ; whilst m the latter, besides the regular mandibles and maxill* there is a variable number of feet-jaws (§53 ).-In the higher part of the Molluscous serW *. a regular gradation may be traced. cav^t ^ i T br ° Ugllt to g ether > to fo rm a pharyngeal tube ; a buccal orifW ^fi aC t at , ltS entrance ; J aws a nd lips are developed at the modSj I * .i cavity; and external members are developed or noaihed, for the purpose of bringing the food within their reach Yet rZ ^J^P^ent of these more special organs does not supersede the iiecessity of those more generally diffused through the Animal kingdom. Although the hand of Man brings his food to his mouth, yet it is by his nps and j aws that his food is taken into its cavity, as in the Herbivorou s Mammalia; and notwithstanding that the muscular apparatus of the mouth propels the food backwards into the pharynx, it only therebv carries it within reach of the pharyngeal constrictors, which then lav hold of it and draw it into the oesophagus, by the muscles of which it is carrier] down into the stomach, precisely as by the tentacula in the Hydra /Ud it is curious to observe, that just as the tentacula of theHvdra no W? make any attempts to grasp an object that touched ^ S when X stomach is already gorged with food ^n ,Wa ivr • ' ,.1 ne in the act of swallowing at the end o? a ful m^ ^TT"* * ^^ from a like want of readiness to ^ W T ' *?"? T*^ t0 reSult ■ one Importmt offlce of the3e fmot . ons . = j* £*»,£*™5S2 M 2 of 164 OF ALIMENT, ITS INGESTION AND PREPARATION. § locomotive, sensorial, and intellectual powers is often required for this purpose. Its introduction into the mouth is an act of volition in Man ; whilst the masticatory movements to which it is there subjected, may be regarded as essentially c automatic in their character (though capable of being controlled and directed by the will), and as closely corresponding with the instinctive actions of the lower animals. The act of ' degluti- tion/ or swallowing, is of a purely 'reflex' nature, being the result of a nervous influence in which neither the will nor sensation is concerned ; for when the solid or fluid contents of the mouth are brought in contact with the surface of the pharynx, the impression made upon the nerve dis- tributed on it is transmitted to the upper part of the spinal cord ; and an automatic impulse is propagated along the motor fibrils, by which the muscular movements requisite for the action of swallowing are produced. A similar action assists the propulsion of food down the oesophagus, and the movements of the stomach seem to be in part excited in the same manner ; but the proper fibres of the oesophagus itself are not dependent for their power of action upon nervous influence, nor are those of the stomach. Beyond the stomach, the connection of the motions of the alimentary canal with the nervous system almost wholly ceases, the ordi- nary peristaltic movements of the intestines appearing to depend upon the stimulus directly applied to their muscular coat by the contact of food ; although they may be in some degree controlled by a system of muscles disposed around the outlet of the canal, which are, like those at its entrance, partly involuntary, and partly under the direction and restraint of the will. — So, in descending the Animal scale, we find that the introduction of food into the digestive cavity is first removed from the agency of the intellect and will, and provided-for by the instincts alone ; this is probably the case in even the highest Invertebrata, and likewise in the lower Vertebrata, as it undoubtedly is in the infantile condition of Man. When we descend to Zoophytes, we find strong reason to believe that the movements of the tentacula, by which the food is grasped and drawn into the stomach, are not merely involuntary, but are not even dependent upon sensation ; and there is still stronger reason to believe that such is the case,^ in those inferior Mollusks in which the supply of food is obtained by ciliary currents. And thus we may perceive a regular gradation between those beings, in which the supply of food has been made (for a wise purpose) dependent upon the highest exercise of the functions of Animal life, and those in which the operation is as purely Organic as it is in Plants. (See chap, xiii.) 148. H educing Apparatus. accessory be noticed, before we proceed to the account of the essential part of the Digestive apparatus, and of the operations to which it is subservient. In order that solid food may be more easily dissolved in the stomach, it is frequently submitted in the first instance to a process of mechanical reduction by a triturating apparatus ; and either at the same time, or at some other, it is incorporated with a Salivary fluid, the admixture of which not only renders the subsequent process of solution more easy, but also appears to effect a change in the alimentary matters themselves. It is in animals that are destined to feed upon vegetable substances, and especially upon such as are of tough consistence, that we find this reduc- ing apparatus most powerful and efficient ; for as the flesh of animals is -** REDUCING APPARATUS. 165 of comparatively easy solubility, there is less occasion for any such, pre- liminary preparation. This reduction is not by any means constantly penormed, however, in the mouth ; for almost any part of the first divi- sion of the alimentary canal may be the seat of the apparatus. — In the -Radiated sub-kingdom, there are none save the typical Echinida, which possess such a reducing apparatus; but it is remarkable that it should attain in that group &n extraordinary com- plexity, and should oc- cupy a situation corre- sponding to that which it possesses in the high- est Vertebrata. The 'lantern of Aristotle' (%. 95, a), as it is sometimes termed, is a five-sided conical mass, composed of five ' jaws' lx i apposition with one another, each of them having the form of a triangular of their sides, are flattened, look towards those of Fig. 95. V V k **;- two which pyramid ; their Anatomy of Echinus lividus, laid open, from the under side ; — a, buccal apparatus turned to one side ; b } portion of tegumentary ,-. . ~~ membrane surrounding the mouth; c, calcareous jaws; d, ceso- tne neighbouring mWS P na g us ; e > commencement of intestine ; /, mesenteric membrane ; — •» t - & J ' 9, duplicature of the intestine, which forms a second convolution, h, along the course of the first, and terminates at the anal orifice i, around which are seen the five oviducts ; j, ovaria ; k k, ambulacral vesicles . and have their surfaces roughened by grooves like those of a file; whilst the third, which is somewhat convex, is external, and serves for }Tl f ^^nt of the muscles that fix the apparatus to the interior of the sneil which is furnished with projections round the oral orifice developed or this purpose. Each jaw contains a long pointed tooth of remarkable hardness, which projects to a considerable length from its socket, and tfiis appears (like the ' tusks' of Mammalia) to be continually renewed by new growth at its base, as it is worn away at its apex. The whole is nmv ed by a most complete set of muscles, which can bring the entire ' lantern' nearer to the oral orifice, and can cause the points of the teeth to proj ect beyond it • which can separate the j aws, and consequently the points of the teeth, from one another, and then draw them together so as to enable the latter to grasp and divide any hard substance adapted for food ; and which can subj ect this to trituration, not merely between the points of the teeth, but also between the flattened file-like surfaces of A IE' M I!™? ^ ^ t0 W ° rk a ^ ainst eac * other.-Among the Acephalous Mollusks, no reducing apparatus seems usually to be required tZl £ g i m a State ° f Ver T fine tiri&m at the time it Src! duced ; nevertheless m some of the Bryozoa, a 'gizzard' or muscular stomach, apparently subservient to this purpose, intervenes between the oesophagus and the true digestive stomacl/ A gizzard of con^ralle power, closely resembling that of Birds, is found in many GaSopods especially m those whose mouth is unfurnished with a LJ^IT^ I i I I 166 OF ALIMENT, ITS INGESTION AND PREPARATION. Fig. 96. B A first ratus ; this is the case, for example, with Aplysia (Fig. 50), whose gizzard (%), situated between the crop (g, g) and the true digestive stomach (h\ is lined with firm horny teeth; and in some instances, these teeth are replaced by firm calcareous plates, adapted to crush the shells of the smaller MoUusks that are devoured by these animals, which is the case m Bulla. ' But m the greater number of instances, the reduction of the food is accomplished by some part of the buccal apparatus ; either by the cutting jaws with which some Gasteropods are furnished; or, as in Buc- cmum by the teeth which form a sort of palatal lining to the proboscis, and which are carried outwards by its eversion; or, as in Patella and some other phytophagous Gasteropods, by the long rasp-like tongue. In the Cephalopoda, notwithstanding the presence of jaws for the division of the food, its reduction seems to be chiefly accomplished by a muscular gizzard closely resembling that of Birds.— In most of these instances, we find salivary glands pouring their secretion into the mouth or pharynx; and where a gizzard exists, there is usually also a 'crop' or stomach' (which is rather to be regarded as a dilatation of the lower part of the cesophagus), into which the food is received in the first instance, and in which it is probably macerated in the salivary fluid before being subjected to the trituration of the gizzard. 149. In the lower Articulata, there is seldom any special re- ducing apparatus ; in the Rotifer a, however, which obtain their food (like the Bryozoa) by ciliary action, we find, in place of a gizzard, a curious pair of jaws (Fig. 96, e), fur- nished with sharp hard teeth, and worked by a complex muscular apparatus; these are situated in the pharynx, and serve to divide the larger particles as they pass downwards to the stomach. , & f^r CD ^■^ I \ #/ Aft '-CIUE^- :l vO- ' t I 'hll II 111 1 V" »il u> > v • 3 9 < U * r 6 J % t I ■/ 9 H # I f * J — UU1 1 1 < ">^ Ae ' ft * f \ £'1 , :/ t r*--r - ■A' H v / i Owing to Vlr* the transparency of these animals, the movements of their jaws can be distinctly seen, when the ' wheels ' are in action, Rotifer vulgaris, as seen at a with the wheels drawn-in, and at b with the wheels expanded :— a, mouth; b, eye-spots; e, wheels; d, siphon; e, jaws and teeth; f, alimentary canal; g y glan- dular (?) mass inclosing it; h, longitudinal muscles ; i, i, tubes of water- vascular system ; Tc, young ani- mal ; I, cloaca. and are driving downwards particles which have entered the mouth. Mandibulate sects, the reduction of the food, as | well as its division and ingestion, is partly performed by the jaws, and the mouth is usually furnished with salivary glands ; but many insects are also provided with a gizzard, which, as in the cases already alluded-to, does ' The Author once met with an entire shell of Pandora rostrata in the gizzard of a Bulla. REDUCING APPARATUS. 167 Of that dental reducing not act upon the food until it has been received into the crop. In many of the higher Crustacea, notwithstanding their complex buccal apparatus, the stomach is furnished with a powerful set of teeth, affixed to a complicated framework which is worked by powerful muscles, so as to constitute -very efficient reducing instruments. Among the mandibulate Articulata, salivary glands are almost invariably found opening either into the mouth, pharynx, or oesophagus ; but sometimes the stomach itself is clothed with caeca, that appear to have a similar character.— apparatus, which is so especially characteristic of the Vertebrata, some notice has already been taken (§§ 60, 64, 72); it may, however, be here remarked that the bones surrounding the mouth are far more copiously set with teeth in Fishes, than they are in the higher animals, for there is scarcely any one of these bones that is not furnished with them in some tribe or other, and they sometimes all bear them at once; among Rep- tiles they present themselves on the palatine and pterygoid bones and the vomer, though they are limited to the j aws in Sauria ; and with the higher specialization which the dental apparatus acquires in Mammals, ^e find it always restricted to the jaws alone. In the class of Birds, the deficiency of teeth is supplied by an adaptation of the stomach for mechanical reduction, that carries us back to the plan so common among the Invertebrata. There is this curious distinction, however, that the gizzard succeeds, instead of preceding, the proper gastric cavity (Fig. 105, d); the food being not only entirely macerated in the salivary and other secretions furnished by the crop, but also impregnated with the proper gastric fluid, before it is subjected to the triturating action of the gizzard. 150. Although in the Mammalia the reducing apparatus is limited to the mouth, yet the stomach frequently presents a complex arrangement, oi which the purpose seems to be to favour the mechanical reduction of the food, and its impregnation with fluid, before it is subjected to the true digestive process. The most remarkable example of this kind is pre- sented in the group of Ruminantia, in which the stomach is subdivided into four distinct cavities; the first two of these, however, being rather dilatations of the oesophagus, than parts of the stomach itself. The solid lood, on its first passage down the oesophagus in a crude unmasticated state, enters the large cavity termed the ingluvies or c paunch ' (Fig. 97, 6), which, like the crop of Birds, serves as a temporary receptacle for it, and moistens it with the fluid secreted from its walls; the liquid swallowed, on the other hand, seems to be specially directed into the second cavity (c), which is termed the reticulum or ' honeycomb- stomach ' from the reticulated appearance of its interior, occasioned by the irregular folding of its internal membrane. It is here that the peculiar provision of * water-cells ' is found, for which the Camel has long been so celebrated, but which exists in a greater or less degree in all the Ruminants. These cells, which are the spaces between the con- deepest reticulations, are bounded by ? muscular fasciculi ; by the traction of one set of which their orifices may be closed and their contents retained ;> whilst by that of another set, the fluid they contain may be expelled into the general cavity of the stomach. After remaining there until sufficiently impregnated with fluid, the solid matters which have passed into the first and second stomachs are returned at intervals m the form of little balls or pellets, to the mouth ; where they undergo a ^^^^^^^■1 i 1 ; 168 OF ALIMENT, ITS INGESTION AND PREPARATION. thorough mastication, and are completely incorporated with salivary fluid Fig. 97. . y ' i ■■ > Stomach of Sheep ; — a, oesophagus ; b, ingluvies, or paunch ; c, reticulum, or honeycomb-stomach ; d, omasum, or many-plies ; e, abomasum, or reed • f, pylorus. Wken ___ f lowed, the food is directed/ in the manner to be pre- sently described, into the third sto- mach (d), the oma- sum, commonly called the ' many- plies' from the pe- culiar manner in which its membrane is dis^ posed ; this pre- sents a number of lining folds, lying nearly close to each other like the leaves of a book, but all directed by their free edges to the centre of the tube, a narrow fold • j • i _ Fig. 98. c i ---:?V:u-- Section of part of the Stomach of the Sheep, to show the demi-canal of the oesophagus; the mu- cous membrane is for the most part removed to show the arrangement of the muscular fibres.— At a is seen the termination of the oesophageal tube the cut edge of whose mucous membrane is shown at b. The lining of the first stomach is shown at e, c; and the mucous membrane of the second stomach is seen to be raised from the subjacent fibres at d. At e, e, the lips of the demi-canal are s ®f. n hounding the groove, at the lower end of which is the entrance to the third stomach or many-plies. ervening broad ones. The food, now re- duced to a pulpy state, has there- fore to pass over a large surface, before it can reach the outlet of that cavity; which leads to the abomasum or fourth stomach (e). commonly called the reed. This is the seat of the true process of digestion, the gastric fluid being secreted from it alone ; and it is from this part of the calf's stomach that the ' rennet ' is taken, which derives its extraordinary power of coagulating milk from the organic acid It contains. In the sucking animal, the milk which forms its nourishment passes directly into this stomach; the aperture leading - being closed, and the folds of the third adhering together so as to form a to the first and second tube. The narrow undivided direction of the food into one or another of these cavities appears to be effected without any volun- tary on the part of the animal itself, but to result simply from the very peculiar endow- ments of the lower part of the oesophagus. This does not entirely terminate at its opening into the first stomach or nannnh Tmt if ;« REDUCING APPARATUS. — DIGESTIVE APPARATUS. 169 continued onwards as a deep groove with two lips (Fig. 98) : by the closure of these lips (e, e) it is made to form a tube, which serves to convey the food onwards into the third stomach; but when they separate, the food is allowed to pass either into the first or the second stomach. When the food is first swallowed, it has undergone but very little mastication ; it is consequently firm in its consistence, and is brought down to the termination of the esophagus in dry bulky masses. These separate the lips of the groove °r demi-canal, and pass into the first and second stomachs. After they have been macerated in the fluids of these cavities, they are returned to the mouth by a reverse peristaltic action of the oesophagus ; this return takes place in a very regular manner, the food being shaped into ^lobular pellets by compression within a sort of mould formed by the ends of the demi-canal, drawn together, and the pellets being conveyed to the mouth at regular intervals, apparently by a rhythmical movement of the esophagus. After its second mastication, it is again swallowed in a pulpy semi-fluid state ; and it now passes along the groove which forms the continuation of the oesophagus, without opening its lips ; and is thus conveyed into the third stomach, whence it passes to the fourth. Now that the condition of the food as to bulk and solidity, is the circumstance which determines the opening or closure of the lips of the groove, and which consequently regulates its passage into the first and second stomachs, °r into the third and fourth, appears from the experiments of Flour ens ;* who found that when the food, the first time of being swallowed, was artificially reduced to a soft and pulpy condition, it passed for the most Part along the demi-canal into the third stomach, as if it had been ruminated, 5 — only a small portion finding its way into the first and second stomachs. lol. Digestive Apparatus. We r in the torms under which the proper Digestive apparatus presents itself different classes of animals; and the mode in which it operates. Putting § Hyd ^^w v X an no ^uiiuiuiuiio ±fc uilcfcU UL11U.C1 WI11UI1 Wt5 JLLI1U It 111 Lilt Actinia, and other solitary Zoophytes. Thus in the Hydra (§ 38), it will be remembered, the stomach occupies the whole of the cavity enclosed by the outer walls ; and the membrane with which it is lined is so completely identical with that which forms the external integument, that the one may be made to take the place of the other without any injury to the animal. Yet it is obvious that a powerfully-solvent fluid is secreted from the walls of the gastric cavity; for the soft parts of the food which is drawn into it (usually in a living state) are gradually dissolved and this without the assistance of any mechanical trituration; whilst the hard parts are ejected again by the orifice through which they were in- troduced. The product of this operation appears to be absorbed from the lining of the digestive sac, by the whole of the tissue which surrounds it; and to be conveyed into the tentacula (which are the only parts not with purpose Hydroida the lower part of the digestive sac of each polype is prolonged into with a ■* Li 'Ami des Sci. Nat." (1832), Tom. xxviii ; and "Memoires de Physiologie Com- paree " 1843. P^^^^^H ■ \ * I I 170 OF ALIMENT, ITS INGESTION AND PREPARATION. Fig. 99. through so that the stem and branches of the entire arborescent structure contain a continuation of the gastric cavities of the several polypes which . they bear (Fig. 99); and the fluid that has been prepared by the diges- tive operations of the latter, the aperture at the lower extremity of each (which is guarded by a sphincter muscle), and is received into the system of ramifying tubes, which may be considered as an extension of the diges- tive cavity throughout the general structure, for the purpose of conveying to every portion of it, and especially to such parts as are undergoing m- Campanularia gelatinosa ; A, upper part of the stem and branches, of the natural size : — B, a small portion enlarged, show- ing the structure of the animal ; — a, terminal branch bearing polypes ; b, polype-bud partially developed ; c, horny cell, con- taining the expanded polype d ; e, ovarian capsule, containing medusiform gemmae in various stages of development ; f, fleshy substance extending through the stem and branches, and con- necting the different polype-cells and ovarian capsules ; g, am nular constrictions at the base of the branches. crease by the formation of new branches or po- lype-cells, the materials prepared for nutrition. Through this system of canals, the contained fluid has been observed to move with consider- able regularity, and in a manner that cannot be accounted - for by any mechanical agency communicated bv the sto- with a particles is seen sufficiently high magnifying power moving along contraction of the machs of the polypes. Thus, when the stem and branches of a living Sertulctria are examined a current of granular the axis; which, after continuing one or two minutes in the same direction, changes and sets in the opposite one, in which it continues about as long, and then resumes the first ; thus alternately flowing down the stem to the extremities of the branches, and back again. The change of direction is sometimes immediate ; but at other times the particles are quiet for a while, or exhibit a confused whirling motion for a few seconds, before it takes place. The current extends into the stomachs of the polypes ; in which, as well as in their orifices, a continual agitation of particles is perceptible.' i t _-. * DIGESTIVE APPARATUS. 171 When these particles are allowed to escape from a cut branch, they exhibit for a time an apparently spontaneous motion. No contraction °f the tube, any more than of the stomach, seems concerned in the pro- duction of the currents ; and their rapidity and constancy appear inti- mately connected with the activity of the nutritive processes taking place in the parts towards which they are directed, the predominant ' set' of the current being towards any portion which is undergoing develop- ment, and away from any part which exhibits a diminution or loss of vitality. In the Tubularia, in which the polype-bearing stem ramifies but little, or not at all, and is sometimes divided by nodes or partitions somewhat resembling those of the Char a (chap, viil), the movement of fluid very strongly resembles that which is seen in that plant ; for the current passes down one side, crosses the septum, and then ascends the °ther side, following a somewhat spiral line of particles deposited on the ^alb of the tube.* 152. The condition of the digestive apparatus in the Actiniform and Alcyonian Zoophytes, does not essentially differ from that just described j save in the fact that the stomachal sac does not occupy more than the cen- tral portion of the body, being surrounded by a visceral cavity which is . subdivided by radiating partitions into chambers containing the generative apparatus. With this visceral cavity the stomach communicates, alike in the Actiniat and in the Alcyonium, by a circular orifice at its base (Fig. 100, d), which is surrounded by a muscular sphincter; and thus the fluid product of digestion, together with water introduced for the purpose of respiration, can pass freely from the stomach into the sur- rounding chambers, and into the cavities of the tentacula. In most species of Actinia (if not in all), the tentacula have pores at their ex- tremities ; and the broad leaf-like tentacula of the Alcyonia have pores m the papillae which fringe them. These pores can scarcely be regarded in the light of anal orifices, the indigestible parts of the food swallowed by these animals (such as the shells of crabs and mollusks) being rejected by the mouth. — In the composite Helianthoid Zoophytes, the several polypes, whilst framed on the same general plan with the Actinia, communicate with each other more or less freely by orifices and passages between their respective visceral cavities. In the Zoanthidce, whose polypes are com- paratively isolated, this communication is established by a stalk-like pro- longation from the base of each, almost as in the compound Hydroida. But in the compound Fungice, Astrece, Meandrince, and other Zoophytes, whose associated polypes encase stony lamelliform corals, the connection is formed by numerous lateral openings : and where the polypes are sepa- rated by intervening fleshy tissue, these openings form a system of passages through it, which must be regarded as continuations of the visceral cavities of the Polypes themselves. So intimate in many * See the Memoir of Mr. Lister < On the Structure and Functions of Tubular and Cellular Polypi m-PMos. Transact./' 1834; and that of Prof. Allman < On Cordylo- phora lacustris, m " Philos. Transact.," 1853. + In thepreyious editions of this work, it was inadvertently stated that the stomach in the Actmiais closed at the bottom. All recent investigators, however, agree in affirming that a definite communication exists between the stomach and the visceral cavity, though the orifice is frequently of small size. Th* A«Tii* a +. ,1*™™+;™ ^ +i„-« m™^ f\ . , 7? size. The earliest description of this structure which the Author has met with, is contained in Mr. Teale's 'Observations on the Anatomy of Actinia Gonacea,' m the << Trans, of the Leeds Philos. Soc," Vol. I., 1836. J M * *• K I I 172 OF ALIMENT, ITS INGESTION AND PREPARATION. polyp scarcely anything but a mouth that can be said to be private property. "* among which the general cavity of the body (Fig. 100, e), into which the Fig. 100. Terminal portion of Alcyonian polype, considerably en- larged and laid open (a) longitudinally, (b) transversely, to show the interior structure j— a, tentacula ; 6, mouth ; ^sto- mach ; d, its inferior opening into the general cavity, of which the upper part is seen at e; f, longitudinal partitions travers- ing the space between the stomaeh and the external parietes • / , prolongations of these, as folds in the wall of the general cavity ; g, canals surrounding the stomach, and communicatin with the cavity of the tentacula ; g', one of the tentacula laiu open ; h, groups of spicules situated at the base of the tenta- cula ; k, filiform appendices. stomach of each Polype opens at its base, communi- cates so freely with that of other polypes of the same polypidom, that the whole assemblage forms a system of canals extending through the mass, very much after the manner of those of a sponge. — This kind of com- munication results, in all the foregoing cases, from the extension of the fabric by gemmation • the visceral cavity of each newly formed part being an offset from that of a previously exist- ing polype; and the con- nection, which is at first extremely free, being never entirely closed. 153. The conformation of the digestive apparatus among the Acalephce does not present advance any the decided Zoo- upon phytic type. In the lower ' composite 5 forms of this group, the plan of struc- ture is essentially the same , . . . " a » in the Compound Hy- droida; the stomachal cavities of the several polypoid bodies being ch other. In the Medusa and thmr in free communication with allies, however, we find an arrangement which more reminds us of the Actiniform type. From the central stomach, which is gene- rally imbedded in the substance of the disk, but sometimes hangs down from its highest point like a pedicle, a set of radiating canals pass off towards the edge of the disk, where they usually communicate with a marginal vessel. mi " "* ' . - — ___ j _ Medusae v -- & . w^ ^ w±±.l^±± they are usually only four in number, and never anastomose with each The simplest arrangement of these ' gastro - other. Med rous at their central commencement, and their number is further greatly increased by subdivision towards the margin of the disk (Fig. 36); On the Structure and Classification of Zoophytes," Philadelphia 1846- * See Dana " pp. 45—48. ** DIGESTIVE APPAEATUS. 173 Fm. 101. frequently, too, they anastomose with each other, especially in their peripheral portion; and sometimes the anastomosis is so free, that this system of prolongations from the stomach forms a complete vascular net- work near the margin of the disk (Fig. 92). — A series of apertures com- municating between the marginal vessel and the external surface, has been described by Prof. Ehrenberg in Cyancea aurita (Fig. 36, e, e); but "the existence of these is doubtful. Should they be really present, they would not be more entitled to be regarded as ' anal pores, 5 than are the orifices of the tentacula of Actiniae. In fact, the gastro-vascular system °f Medusae may be likened to the radiating chambers into which the yisceral cavity of Actiniae is subdivided ; the chief difference being, that its cavities are lengthened-out in accordance with the expansion of the disk. There are links of connection, in fact, which establish the transi- tion from one form to the other. 154. The same type is exhibited under a higher form and more com- plete development, in the Digestive apparatus of some of those more elevated members of the Radiated series, which constitute the class ^chinodermata. In the Ophiurida (Fig. 8) and ^erm^ (Fig. 37), the gastric cavity is still a single sac, occupying the centre of the body, and furnished with but one orifice, through which food is drawn in, a &d indigestible or fecal matter is ejected. But the sac is now com- pletely cut off from the general cavity of the body, in which it is freely suspended. In the caecal pro- longations of the central stomach of the Asterias, and in the abun-. dance of the cells which line them, "we find the first manifestation of special glandular organs for the elaboration of the fluids required m the digestive processes, though the precise nature of these is as yet unknown. The proper digestive apparatus appears to be limited in the Star-fish, as in the Ophiura, to the central cavity; and being re- moved by a considerable interval from other parts of the body, it does not here directly convey the nutritious food to the tissues which make use of it; and the interven- I tion of a Set of vessels becomes Structure of Polycelis levigatus (one of the Pla requisite, for its absorption and ^^UXA^Tt^XS^^ transmission to distant parts. d J st °mach; e, ramifications of gastric canals The peculiar manner in which ^^ the lowest members of the Ar- ?ana ^ k> k > J ldl \ cts ' ^ lat ^ ion at their point of u • £XJ - junction ; m, female genital orifice. 174 OF ALIMENT, ITS INGESTION AND PREPARATION. ticulated series, constituting the Cestoid tribe of Entozoa, obtain their nutriment, renders it unnecessary, as already remarked (§ 138), that they should be furnished with a digestive cavity; and in the higher group of Trematoda, as also in the tribe of Planarice, which (if not actually included within it) is closely allied to it, we still find the orifice (Fig. 101, a), which must It is a remarkable feature in stomach furnished with but a single as mouth and serve equally anus. lacing the structure of these animals, that from their principal stomach (d), a series of ramifying caeca are prolonged through the whole substance of the body. These do not lie loosely in a visceral cavity, but seem channelled out, as it were, in the midst of solid parenchyma \ but it is affirmed by Quatrefages that a visceral cavity really exists, and that the digestive sac is tied to its walls by bands and filaments inter- in every direction, so as to form a kind of areolar tissue that fills up the intervening space. — On nearly the same grade with the preceding, as regards the conformation of its digestive apparatus, we may place the curious Rotiferous Animalcule, of which the male has been already noticed as entirely agastric (§ 137): for the female possesses neither intestine nor anal orifice, and must ej ect the indigestible particles of food through its mouth. These are the only known instances among the Articulata, in which the digestive cavity has but one orifice. — No conformation of a similar kind is ever met- with, either in the Molluscous or in the Vertebrated series. 155. We have now to glance at those higher forms of the digestive apparatus, in which a second orifice or ' anus' is present, for the discharge of indigestible or fsecal matters ; and this, as we shall see, is by far the most general plan of conformation. Although this orifice may externally T be situated in the neighbourhood of the mouth, yet it always communi- cates with the part of the digestive cavity that is most remote from it; this cavity being usually more or less prolonged in the form of a tube. Fig. 102. A \V.' I -■• A. $\' : -\% "''litr-Jhcz ■■> fit*!' # . HI t \ y.( fj FK ''M ;, y iff S IKHPi rSr* }Mm h i 7 1;-, -t - : A, Cgdijppepileics.—B, Beroe Forskalii, showing the tubular prolongations of the stomach. The lowest animals which present this great advance in the type of confor- mation, are the Ciliograde Acalephce (Fig. 102), which in this important DIGESTIVE APPARATUS. 175 Fig. 103. x B C aft . : particular differ widely from the rest of their class, so that some Zoologists remove them from it altogether. As in the Medusae, we find the deficiency of blood-vessels supplied by tubular prolongations of the gastric cavity (b), "which extend themselves into the substance of the tissues, and which convey to them the materials for their growth and renovation. In the yydippe (a) we see the first indication of that division between the ' stomach' and intestinal canal/ which be- comes so much more obvious and important in the higher classes. This division is by no Cleans well marked, however, oven in the highest Radiata j the alimentary canal, which commences with the mouth and terminates at the anus, being of nearly uniform size throughout, and apparently of similar endowments ; as we s oe in the Echinus (Fig. 95), and the Holothuria (Fig. 40.) In the Nematoid Entozoa, gam, the same uniformity Presents itself (Fig. 52) ; and rt is curious to observe this almost exactly repeated in the Serpent tribe, whose or- ganisation is so much higher ; but the prolongation and uni- formity of whose body seems to necessitate a somewhat / I A A i % \ \ ap- i d Digestive apparatus of Annelida : — a, Aphrodite acu- leataj a, mouth; b, fleshy proboscis; c, central portion of digestive tube, representing the stomach; d, lateral appendages; e, anus. — b, Hcemopis vorax ; a, mouth- b, lateral sacculi of the stomach; c, two large terminal caeca; d, intestine :— c, Aulostoma niqrescens : a mouth- 6, cseca ; c, intestine. ' 9 similar conformation of the alimentary canal. Even in the Annelida, the ral plan is continued with but little modification; the ali- mentary canal passing in a straight line from one ex- tremity to the other, without distinction of stomach or in- testine (Fig. 103); but from its sides we find c^cal prolongations ex tending, sometimes as a single pair of prolonged tubes (c, b)* sometime* as mere sacculi in the walls of the stomach (b, 6), and sometimes as a multiplied and extended series of appendages (a, d), which seem, like the radiating («ca of tne Asterias, to be rather glandular in thet character, than destined to admit the passage of food into them. 156. From these forms we might gradually ascend towards the more camp ex digestive ^apparatus of Insects and Crustacea; but we shall first revert to that of the lowest Mollusca, as the simplest case in wHch the division of the canal into stomach and intestine is clearly malked out ■^^■^ ■ 176 OF ALIMENT, ITS INGESTION AND PREPARATION. (relation into distinct glands which dis- * This is well seen in the Bryozoa (Fig. 49), in which, as in the Hydra, the whole process of digestion may be watched, owing to the transparency of the tissues of the animal. The digestive stomach (c) is here separated from the comparatively narrow intestinal tube, by a valvular orifice (/), or true * pylorus ;' and whilst the whole process of solution takes place in the former division of the canal, whose walls are beset with secreting follicles (h) that pour forth a bile-like fluid, the latter seems to have little to do, save to convey to the anal orifice (I) at the mouth of the polype- cell the excrementitious particles which are to be ei ected from it. The w i the whole of the Molluscous series : the principal advance being shown (Fig. 50) in the higher deve- lopment of the liver, by the withdrawal of its follicles from the parietes of the stomach, and by their a charge their secretion into the upper part of the intestinal tube ; in the development of a rudimentary pancreas (in the Cephalopoda) ; and in the increased length given to the intestinal portion of the tube, which usually makes several i convolutions' before it finally terminates at the anus. — The same is the mode of advance which presents itself in the higher Articulata ; among which the digestive apparatus of the Crustacea (Fig. 58, c, d,e) presents the greatest resemblance to that of Mollusca, whilst that of Insects (Fig. 57, c, d, e) is remarkable for the very low grade of development of the liver, which only makes its appearance in the form of a small number of csecal tubuli (/, /) discharging their contents into the intestine. As yet no distinction between the ' small' and the < large' intestine presents itself; this being only manifested fully in the higher Yertebrata. 157. It is interesting to observe that, among the higher tribes of both the Molluscous and the Articulated sub-kingdoms, we should find examples of reversion to that lower type of conformation of the digestive appa- ratus, which is manifested in its extension into c^ecal prolongations that radiate into parts of the body remote from the proper digestive sac, and in the diffusion of the glandular apparatus over the whole or the greater part of their surface. This is the case with the Nudibranchiate Gastero- pods (Fig. 104), in which the alimentary canal sends off as many cseca as Fig. 104 JEolis Inca, a Nudibranchiate Gasteropod, that of the Planarice (Fig. 101), each one of them, however, terminating in one of the dorsal papillae, and being there surrounded by a group of hepatic cells, the liver being thus subdivided (so to speak) amongst them all. So among the Crustacea, we find the curious group of Pycnogonidce especially characterized by the extension of the digestive cavity into the i DIGESTIVE APPARATUS. 177 articulated members, almost to their extremities (Fig. 105), and by the s preading-out of the hepatic cells over the entire surface of these cseca ; Fig. 105. Fig. 106. w^ Ammotheapyenogonoides ; — a, narrow oesophagus ; b, stomach : c, intestine ; d, digestive caeca of the feet-jaws ; e e, digestive caeca of the legs. Digestive apparatus of My gale : — a, cephalic ganglion; 5, bifurcated origin of oesophagus ; c f stomach with lateral caeca passing into the limbs and palpi ; d, portion which traverses the abdominal peduncle; e, dilated intestine, receiving biliary vessels ; f 9 small intestine ; g, caecal enlarge- ment, receiving the urinary vessels h 3 h. and it is peculiarly worthy of note, that whilst the alimentary matter passes into these extensions, and is conveyed by a continual flux and re- flux through them (as microscopic observation shows) into the immediate neighbourhood of every portion of the body, the special circulating appa- ratus presents its very lowest grade of development (§ 231). An approxi- mation to the same structure is seen in the csecal prolongations of the digestive cavity, which pass into the thoracic members of the Arachnida (Fig. 106, c); these, however, would not seem to possess any special secret- ing function, a distinct biliary apparatus being found in the abdomen. But in certain Acaridce we return to a condition even lower than that of the Pycnogonidse ; the parietes of the caecal prolongations of the digestive cavity not being distinctly separable from the tissues around, and not even the rudiment of a proper circulating apparatus being distinguishable. b 158. The digestive apparatus of Vertebrated animals may be consi- dered as carrying onwards the general plan which has been seen to pre- vail m the Molluscous sub-kingdom. Thus in Fishes, there is usually a well-marked distinction between the stomach and the intestinal tube; the N P^V . 178 OF ALIMENT, ITS INGESTION AND PBEPARATION. * liver attains a high development, and is completely withdrawn from the parietes of the alimentary canal; and the also. begins to large The intestinal pancreas, e w appear as a distinct gland, instead of being a mere collection of cseca clustering around the digestive cavity. In the Amphioxus, however, we see a reversion to a very inferior type; the digestive portion of the alimentary canal (Fig. 127, i, i) being a short tube of nearly uniform size throughout, which commences behind the pharyngeal respiratory apparatus, and runs backwards almost in a straight line to the anal orifice, receiving in its passage the fluid discharged by the simple large csecum, which is the only rudiment of the liver. In the Cychstomi the stomach is merely a dilated portion of the tube, which lies in a straight line between the oesophagus and the intestine ; but in nearly all the higher Fishes, both Osseous and Cartilaginous, it forms a cavity (Fig. 126, 2 ) which lies out of the course of the alimentary canal, and is adapted to retain the food while the process of solution is being carried on. It is usually separated from the intestine by a narrow 1 pyloric' orifice ; and it is also generally divided from the oesophagus by a ' cardiac 5 valve, the aperture of which, however, is very wide, so as to admit the food which is swallowed in an undivided state, and does not completely prevent its regurgitation. Indeed it seems common for Fishes to disgorge the shells and other indigestible parts of their food through their mouths, like Polypes ; and there are some (especially of the Carp tribe) in which a sort of rumination takes place, the food being sent back to the pharynx to be masticated by the pharyngeal teeth, and then returning to the stomach to undergo its final digestion, tube is usually short and almost destitute of convolutions, as well as of nearly uniform diameter throughout, so that the distinction between the small and the large intestine is only marked (and this not constantly) by the ileo-colic valve. The surface of the mucous membrane lining the canal is considerably extended, however, by being thrown into rugce or wrinkled folds, more or less projecting; these in the Osseous Fishes have seldom any great regularity of arrangement; but by the great projection and spiral continuity of one of these folds, the real length of the intestinal tube is greatly increased in the higher Cartilaginous Fishes, just as a spiral staircase is much longer than the cylindrical cavity within which it winds. — In the omnivorous tadpoles of Batrachicm Reptiles, the alimen- tary canal is of great length, and forms numerous convolutions in the abdominal cavity; but the stomach is narrow and elongated, the intes- tinal tube is of nearly equal size throughout, and the boundary between the small and the large intestine is not distinctly marked. This condi- tion is retained in many of the Perennibranchiata ; but in the adult condition of the Frog and its allies, a nearer approach is exhibited to the character presented by the alimentary canal in the higher Reptiles, most of which are carnivorous. In these we usually find the gastric dilatation larger, and more completely distinguished from the intestinal tube ; although the intestine is relatively much shorter, yet the extent of its mucous surface is augmented by rugce and villi; and the small intestine is now more constantly separated from the colon, by a valvular constriction. It is in the vegetable-feeding Turtles, that we find the intestinal tube possessing the greatest length, and presenting the greatest difference of diameter in its two divisions ; and in most of these, as well DIGESTIVE APPARATUS. 179 a « in some other Reptiles, the large intestine has a csecal dilatation at its commencement. ,, . ^ n -Bwds, the length of the alimentary canal varies greatly in the different families, in accordance with their habitual J -~ x *-— - greatest in the granivorous and diet, being Fig. 107. frn - . -vuu WlUCDj CtllU. ICCtOU ±11 tilt? predaceous: there is, however, a certain general plan of conforma- tion, to which the variations may be all referred. In nearly all the members of this class, the food is delayed in a receptacle formed by the dilatation of some part of the mouth, pharynx, or oesophagus, before it passes into the stomach ; and in this receptacle it sometimes undergoes a sort of preliminary digestion. In the Pelican we find it in the form of a wide pouch suspended from the lower jaw, the two halves of which bone are not united at the symphysis, so that the pouch is enormously dilatable ; m the Swift and other birds that catch insects on the wing, a similar extensibility exists in the mem- branous wall of the fauces; in the Cormorant and other fishing ?• i lt 1S the wide esophagus which serves as the receptacle; but in the Granivorous birds, as yell as in many others which take m a large quantity of food at once, ^ find a pouch termed the ' crop' (fig. 107, 6), developed from the side of the gullet, into which the Jood passes when it is first swal- lowed, and in which it lies for some time ; during which it is moistened Digestive apparatus of Common Fowl:~- a oosc Dy a Secretion Copiously poured Out P ha £ us > h meluvies or crop ; c, proventriculus : from glandular follicles in its walls, t O&Z] tS* fi±?ft 'c&Tfffi and is thus prepared for the fur- mtestme > m > m > ureters ; n, oviduct ; , cloaca. ther stages of the digestive process. Below the crop, the oesophagus is usually again dilated, before its termination in the gizzard, into Xh£ cavity, known as the < proventriculus' (c), from the walls of which the true gastric fluid is poured out upon the food that is delayed in it this may be considered as the first or cardiac portion of the true t'astr o Z&*th$£? T Zard \ {d) - ° r mUSCukr sac whi <* BucceedTit ^ S wTt 1 Z^ 7 T P - yl ° nC P0rti0n - The g izzard is » s ™lly a some what lengthened sac, having its intestinal orifice, as well a/thaTbv which it communicates with the proventriculus, at its upper part I n n 2 i$S „ :.•.'';■ /. ft ■' ■» I 1 180 OF ALIMENT, ITS INGESTION AND PREPARATION. th ose whose food requires the aid of mechanical reduction for its solution, the walls of the gizzard are very thick and muscular, and its fibres radiate from two central tendons situated on opposite sides of the cavity, which is lined by a horny epithelium of remarkable toughness. ~ The triturating power of the gizzard is frequently much increased by the presence of hard angular stones, which the bird occasionally swallows, and which are retained in its cavity. In Birds, however, whose food consists of flesh, fish, succulent fruits, or other substances easily reduced, the muscular struc- ture is so little developed, that the original character of the ' gizzard' is en- tirely wanting. The intestinal tube varies considerably in length, and also in the degree of definiteness of the division between the small and the large intestines. At no great distance from the stomach, it receives the secretions of the liver (e) and of the pancreas (g) ; the former gland, which (/) excretory by which the bile can be poured direct into the intestinal tube ; the latter, which is now much increased in size, and has attained a higher grade of glandular development, still pours forth its products, sometimes by two ducts, and in other cases by three. After a larger or smaller number of convolutions, the intestinal tube terminates in the cloaca (o) ; first, however, receiving the products secreted from one or two caecal appendages (k) whose opening into the canal is considered to mark the boundary between the small and the large intestines (i and I). with Mam tribes are adapted to exist ; and no part exhibits a greater diversity of conformation than does the stomach, — even putting aside that peculiar apparatus which has been already described (§ 150) as concerned in the preparation of the aliment in the Euminantia. Thus in the carni- vorous and insectivorous Mammals, whose food is easy of solution, and consequently requires to be but little delayed for the digestive operation the stomach is usually simple in its form, being a mere dilatation of the alimentary canal, and lying nearly in the direction of its course ; but in proportion as the food departs more widely in its composition from the body itself, and is less capable of reduction by the gastric fluid do we find (in most cases at least) the dimensions of the stomach increasing and its form undergoing alteration ; so that it more and more presents the character of a diverticulum, into which the aliment is turned aside and in which it is retained during the time required for its subjection to the solvent power of the gastric juice (Fig. 62, g). The difference in form is principally given by the increased development of the large or left- hand extremity of the stomach, which thus becomes a sort of csecal dila- tation off the line that connects the cardiac and pyloric orifices ; and not unfrequently this portion of the stomach is separated from the other by an incomplete partition. In addition to this, numerous smaller cseca are frequently developed in connection with the principal cavity ; whilst in many Cetacea we find a succession of constrictions intervening between the principal gastric caecum and the pyloric orifice, which partially divide the entire stomach into numerous cavities, all of them to be traversed by the food during its passage from the oesophagus to the intestine. The small intestine is chiefly remarkable for the great length to which it ^^^^^^^^H IP! DIGESTIVE APPARATUS. 181 Mammal its mucous surface by ' valvulse conniventes,' which are deep folds of membrane, succeeding one another at tolerably regular intervals, but not embracing above two-thirds of the circumference of the tube. In nearly all Mammals, save in the Cetacea (which exhibit a return to a fish-like inferiority in this respect), a well-marked distinction between the small and the large intestine is manifested, not only by the presence of the ileo-csecal valve, but also by that difference of diameter from which these two divisions of the canal derive their names. The large intestine is here not only larger in proportion to the small, but is longer, than in Mammals, as in Man character is given to the colon by the peculiar arrangement of its mus- cular bands, so that the extent of its lining membrane is greatly increased. The csecal enlargement, in which the ileum terminates and the colon commences, is frequently of great size, especially in herbivorous animals; being sometimes larger than the stomach, as is the case in the Horse : or even many times larger, Rodentia. may be seen in some of the The villi (Fig. 108), which in all the lower Vertebrata are dispersed over the surface of the large as well as of the small intestine, are now limited to the latter ; whilst from the great number of glandular contained in the former, and from the change in the character of the contents of the alimentary canal which shows itself when they arrive there, it seems probable that whilst the small intestine is the part of the canal in which A bsorption specially takes place, the large is particularly destined for Excretion. 161. Thus then, we see, that although the development of the Digestive apparatus is subj ected, perhaps more than that of any other portion of the apparatus of Organic life, to variations which are immediately con- nected _ wxth the purposes to which it is to be subservient, a gradual specialization can be most distinctly traced, when we take a general survey of the succession of forms which it presents among the principal groups of animals. For, in its lowest condition, we have seen it to consist of a simple cavity excavated in the tissues for the reception of alimentary substances, which, when reduced to the state of solution (chyme), are taken up from every part of its walls, and find their way, without any special system of canals, into the parts of the body most remote from it : whilst the indigestible matters are ej ected by the same orifice as that by which the food was taken-in. Next, we find the digestive sac furnished with parietes of its own, and suspended within a visceral cavity, with which it is at first in free communication; going a step further, we find this communication closed, but the digestive sac itself extended into remote portions of the organism; and, in still hioher forms, the liquid (chyle) which transudes into the visceral cavity, instead of the immediate product (chyme) of digestion, is that which is applied to the purposes of nutrition. Next, we find the digestive sac with its single orifice changed into a canal, provided with a second orifice, through whfch the indigestible and fecal matters are ejected. Next, we find the stomach more or less distinctly separated from the intestinal tube : and the former becomes possessed of special endowments which are not shared b y the latter the gastric fluid being secreted by its walls alone, and the secretion of the liver being either poured into it, or into the first portion m i I 182 OF ALIMENT, ITS INGESTION AND PREPARATION. of the intestine. Next, we find the mucous surface of the intestinal canal considerably extended by folds and duplicatures ; and these are thickly set with villi, each of which not merely contains a copious net- work of blood-vessels, but also includes the commencement of one of the Next, and the large intestine special absorbents ^ or 'lacteals' peculiar to Vertebrated animals, find the distinction between the small we gradually becoming more clearly marked, and each part by its own peculiarities ; whilst at the same time, the concentration of structure is still further indicated by the greater extension of the mucous surface within the canal, by the augmented number of villi in the small intestine, and by the increase of the glandulse and follicles in the large. 162. Now in its general outlines, the history of the Embryonic development of the Digestive apparatus in the higher animals closely corresponds with this; but the entirely different mode in which the alimentation of the embryo is to be accomplished, involves a considerable variation in the particular method adopted. The embryonic cell, when it distinguishable of a store of nutriment (the yolk) provided for it by the parent ; and this it begins to draw-in, like the simple cells of the lowest Entozoa, which it resembles its conditions of existence (§ 138), by The successive broods of cells produced by fissiparous multiplication, are at first nourished after the imbibition through its cell- wall. same fashion ; but these soon tend to spread themselves in the form of a mem- branous expansion around the yolk, and thus to include it completely within the embryonic cavity. This cavity may be likened to the stomach of the Hydra, in every respect save that it has no orifice ; but this is not needed, since it is already filled with the requisite supply of aliment The further introduction of this into the embryonic tissues, is accomplished at first by the cellular layers of the germinal membrane ; but blood-vessels are soon developed, which thenceforth are the special channels for its reception and conveyance. This cavity, however, serves but a temporary purpose,— in this respect corresponding with the cotyle- dons, by which nutriment is ^ received into the embryo of the higher plants; and the permanent digestive apparatus commences in the more advanced embryo, as a small portion of the temporary vitelline sac, that is gradually detached from it, like the stomach of the budding Hydra from that of its parent. The form which this detached portion at first exhibits, is that of a simple tube, closed at both extremities, whilst its middle portion remains connected with the yolk-bag. An oral and an anal orifice are then formed; and the tube thus comes to present the characters of the alimentary canal of the lower Articulata. The next grade in development is the evolution of the stomach, which first shows itself as a projection of the tube towards the left side; and the accessory glands, the liver and pancreas, soon make their appearance, reminding us in their earliest condition of the grade of development which they permanently exhibit in the lower animals (chap, ix.) The short straight tube gradually increases in length, and is thrown into convolu- tions, that part being most increased in length which remains of the smallest diameter; and thus arises the difference between the small and the large intestine. The folds of the mucous membrane, the villi, and the glan- NATURE OF DIGESTIVE PROCESS. 183 dular apparatus, which are the parts most restricted to higher animals, are the last to appear in the course of their development. 163. We have now to consider the proper Function of Digestion, which xn&y be said to commence with the introduction of the food into that part of the alimentary canal, in the walls of which is secreted the fluid destined for its solution. In the higher animals, this secretion is re- stricted to the gastric cavity or stomach; and hence it is named the gastric juice.' The only chemical change which the food appears to have undergone, before being submitted to this, is that which is effected by the admixture of Saliva in the mouth ; a peculiar animal principle contained in this fluid having the power, like the diastase in Plants, of converting starch into sugar. The process thus commenced in the mouth, is retarded in the stomach, the acid character of the gastric fluid being unfavourable to it; it is recommenced, however, in the intestinal canal, after that acid has been neutralized by the alkali of the bile and pan- creatic fluid. The great purpose of the gastric digestion appears to be, to dissolve the albuminous and gelatinous constituents of the food; and, by the withdrawal of these, the remainder are usually reduced to a state of fine division, in which they are afterwards more easily acted-upon. In regard to the constitution of the gastric juice, there is at present much diversity of opinion ; and it does not seem improbable that it may vary in different animals. It appears essentially to consist, however, of a free acid (either the hydrochloric, acetic, or lactic, but generally the first) which is the real solvent ; and of an animal principle in a state of change, which acts as & ferment, and disposes the organic compounds to solution. Water slightly acidulated with these acids, is capable of dissolving albu- minous compounds with the aid of a high temperature ; but if a solution oi ' pepsin' (which is the animal matter obtained by macerating the stomach of a pig in cold water, after it has been repeatedly washed) be added, the acid solvent will then be able to act efficaciously at the ordi- nary temperature of the body. The solvent action of the gastric juice is aided by the movements of the walls of the stomach, which are pro- duced by the successive contractions and relaxations of their muscular fibres. The purpose of this motion is obviously to keep the contents of the stomach in that state of constant agitation, which is most favourable to their chemical solution ; and particularly to bring every portion of the alimentary matter into contact with the lining of the stomach, so as to subject it to the action of the solvent fluid which is poured forth from it. Thus, little by little, the reduction of the alimentary materials to the homogeneous pulpy mass termed chyme is accomplished ; this escapes into the intestine, as fast as it is formed, through the pyloric orifice, which closes itself to solids, but allows liquids to pass ; whilst the solid residue is continually subjected to the same action, until its solution has been effected. There is no doubt, however, that a portion of the nutritious matter dissolved by the gastric fluid is at once absorbed into the blood- vessels of the stomach, without passing either into the intestinal tube, or into the special lacteal system of vessels.— That the action of the gastric juice is in all respects one of a purely chemical nature, there can be no longer any question. When drawn direct from the stomach of Man or of the lower Mammalia, it is found to possess the power of dissolving various P^^H 1 I I !' I I HI 184 OF ALIMENT, ITS INGESTION AND PREPARATION. kinds of alimentary substances, provided that these are submitted to its agency at a temperature equal to that of the body, and are frequently agitated. The solution appears to be in aU respects as perfect as that which is naturally effected in the stomach ; but a longer time is required to make it —a difference which is easily accounted-for, when the impos- sibility of fulfilling all the conditions under which gastric digestion takes place, is borne m mind. The quantity of food which a given amount of gastric fluid can dissolve, is limited; precisely as in the case of the acidu- lous solution of 'pepsin,' or < artificial gastric juice,' whose solvent power is chiefly regulated by the quantity of acid which it contains, the same quantity of pepsin being capable of ' disposing' the solution of many successive amounts of the substance to be acted-on, provided that acid be added as required. 1 64. The Chyme which passes into the intestinal tube, is commonly a greyish, semifluid, and homogeneous substance, possessing a slightly acid otherwise When oily character, the chyme possesses a creamy aspect ; but when it has contained a large proportion of farinaceous matter, the chyme has rather the appearance of gruel. The state in which the various alimentary principles exist in it, has not yet been accurately determined ; the fol- lowing, however, may be near the truth. The albuminous compounds, whether derived from the Animal or from the Vegetable kingdom, whe- ther previously possessing the form of fibrin, of casein, of glutin, &c. , are reduced by solution to the condition of Albumen ; but a part of these compounds may still remain undissolved in the chyme of the intestine Gelatine will be dissolved, or not, according to the previous condition of the substance containing it ; for if this be a tissue which does not readilv yield gelatine to hot water, the gastric fluid will have little influence UP °? ?; / g^my matters of plants are dissolved, when they exist in a soluble form ; but starch is not changed unless it has been previously acted-upon by the saliva, save in the case of the Granivorous Birds and Ruminant Mammals, m which the provision for the mechanical re ■ —M I I I I MM i ■ ^B & its/ When through are separated by a septum composed of animal membrane, the endosmotic current will be from the water towards the alcohol, because the former liquid most readily c wets ' the membrane, and consequently tends most strongly to occupy its capillary passages : but on the other hand, when the separation is made by a thin lamina of caoutchouc, the endosmotic current is from the alcohol towards the water, because the readily imbibed by the septum. former Matteucc that when an organic membrane is employed as a septum, the rapidity of transmission is considerably affected by the direction in which the endos- motic current traverses the membrane. Thus when the skin of the Torpedo was employed, with a solution of sugar on one side of it and curr the water to the sugar, yet this current was strong enough to raise the in- PHYSICAL CONDITIONS OP IMBIBITION. — ENDOSMOSE. 191 terior liquid to 80 degrees, when the water was in contact with the internal surface of the membrane, in the same time that was occupied by its rise to 20 degrees, when the external surface of the membrane was turned towards the water. Again, when the mucous membrane of the stomach °i a dog was used as the septum, and its external (or muscular) surface ^&s placed in contact with alcohol, the passage of water from the other side took place with such rapidity, as to raise the liquid in the tube to 130 degrees ; whilst if the internal (or mucous) surface of the membrane was placed in contact with the alcohol, and the muscular surface with water, the current was only sufficient to raise the liquid 6 degrees in the same time : so that it is evident that the transudation of water takes place much more readily from the mucous lining of the stomach towards the outer side of the viscus, than in an opposite direction, in virtue simply of the physical properties of the membrane. In fact, according to Prof. Matteucci, the cases are very rare in which, with fresh membranes, endos- mose takes place with equal readiness, whichever of their two sides is ex- posed to the water. The direction which is most favourable to endosmose through skins, is usually from the internal to the external surface, with the exception of the skin of the frog, in which the endosmotic current, in the single case of water and alcohol, takes place most readily from the external to the internal surface. But when stomachs and urinary blad- ders are employed, the direction varies much more in accordance with the nature of the liquids employed. This variation appears to have some relation to the physiological conditions in which these membranes are placed in the living animal ; thus the direction most favourable to endos- mose between water and a saccharine solution, is not the same for the stomach of a Ruminant as for that of a Carnivorous animal : as yet, how- ever, no general statement can be made on this subject. When mem- branes are employed, that have been dried or altered by putrefaction, we either do not observe the usual difference arising from the position of the surfaces, or endosmose no longer takes place; thus affording another indication that it is to the physical condition of the perfectly-organised membrane, that we are to look for many of the peculiarities which are noticeable in the transudation of fluids through them. The Exosmotic current does not bear any constant relation to the endosmotic, as may be easily comprehended from the preceding explanation ; for if the liquids have a strong tendency to mutual diffusion, and the difference in the attractive power which the septum has for them respectively is not great, each may find its way towards the other, and a considerable exosmose may ensue, with very little change of level. The amount of the ex- osmotic as of the endosmotic current, varies with the direction in which it traverses the membrane ; thus when sugar, albumen, or gum was em- ployed in solution, its transudation towards water took place most readily from the internal towards the external surface of all the skins examined by Matteucci ;— a fact which is not without its significance, when it is remembered that it is in this direction that the secretion of mucus takes place on the skins of fishes, frogs, &c. 172. Applying these considerations to the phenomena of Imbibition of liquids into the tissues and canals of the living body, we shall have to enquire how far they are capable of being explained on the physical principles which have been now brought forwards.— It has been main- [i I ''' !, ! , I ! 192 OP ABSORPTION AND IMBIBITION. tained by some that Absorption is a purely vital operation, because it does not occur save during the continuance of life. But this is not true; since imbibition will take place into dead tissues, though more slowly than into the same parts when living • and the difference of rate seems to be fully accounted-for, by the difference of the condition between a mass of tissue all whose fluids are stagnant, and another in which an active circulation is taking place. Thus it is stated bv gree. Matteucci hind-legs of a frog recently killed be immersed for some hours in a solution of ferrocyamde of potassium, every part of the viscera will be found to be so penetrated with the salt, that by touching it with a glass rod moistened with a solution of the chloride of iron, a more or less deep blue stain is the result. Now the same effect is produced much more speedily in a living frog^ and it is easily proved that the imbibition takes place, in the latter case, into the blood-vessels, and that the salt is conveyed to the remoter parts of the body by the circulation, instead of having slowly to make its way by transudation through the tissues, as in the dead animal. — But further, not only does the movement of blood in the vessels pro- mote the diffusion of liquid which has been already absorbed ; it also increases the rapidity of the absorption itself in a very extraordinary de- Thus, if a membranous tube, such as a piece of the small intes- tine or of a large vein of an animal, be fixed by one extremity to an opening at the bottom of a vessel filled with water, and have a stopcock attached at the other extremity, and be then immersed in water acidu- lated with sulphuric or hydrochloric acid, it will be some time before the acid will penetrate to the interior of the tube, which is distended with water ; but if the stopcock be opened, and the water be allowed to dis- charge itself, the presence of the acid will be immediately discovered (by tincture of litmus) in the liquid which flows out, showing that the acid has been assisted in its penetration of the walls of the tube, by the passage of a current through its interior. Thus the continuance of the Circulation is odiously one of the most potent of all the conditions of Absorption : and the difference m the rate of the process in dead and living organisms placed under the same circumstan ces, may be accounted-for in great part, if not entirely, by the stoppage of the circulation in the former. All the circumstances which are laid down by Physiologists as favouring Absorp- tion, are in strict accordance with the Physical principles which have been now explained. These circumstances are: — 1. The ready miscibilitv of the liquids to be absorbed with the juices of the body. 2. The penetra- bility of the tissue through which the absorption takes place. 3. The absence of previous distension in the tissues or canals towards which the flow takes place. 4. The elevation of the temperature, within certain limits. 5. The vascularity of the tissue, and the rate of movement of the blood through the vessels. — And the results of experiments upon recently- dead membranes, which retain almost exactly the same physical condi- tions as those which they possessed during life, but have entirely lost their vital properties, seem most decidedly to indicate, that the relative facility with which different substances are absorbed, and the direction most favourable to their passage through the tissues, are determined in great part by the physical relations which those tissues (and the vessels that traverse them) bear to the liquid which is seeking to enter them. In this way, then, many of the phenomena of selective absorption are pro- ABSORPTION IN CELLULAR PLANTS. 19° bably to be explained, especially in Plants and the lower Animals : and others will be shown to be due to the endowments of the cells which are m relation with the Absorbent vessels at their origin. 2. Absorption in Vegetables. 173. In the lowest orders of Plants, we find this function performed "under its most simple conditions. Their substance is composed of vesicles more or less firmly united to each other, and but slightly altered from their original spheroidal form ; and the envelope which surrounds them can seldom be regarded as a distinct structure, generally differing but little from the remainder of the cellular tissue. In all the Algce (§ 24), the whole surface appears to be endowed with the power of absorption to nearly an equal degree; and although the semblance of a stem and roots presents itself in the higher orders, yet these seem to have no other func- tion than to give the means of attachment to the frondose expansion. The preference of particular species of Algae for particular rocks, cannot be fairly considered to indicate that any special absorption of the mineral particles takes place through their roots ; it is much more probably due to the fact, that the materials of these rocks are in some degree diffused by solution through the neighbouring water : and something may be also attributable to the mechanical qualities of the rocks, as affording advan- tageous surfaces for attachment. — The difference in the situation inha- bited by the Lichens (§ 25) appears to involve a separate appropriation of portions of their surface to the nutritive and reproductive functions, and a ce rtain specialization of the absorbing organs. The upper surface of these plants, being exposed to the sun and air, becomes hard and dry, a condition which seems to favour the evolution of the fruit ; whilst it is mostly by the lower surface, which is usually soft and pale, that the nutriment is introduced into the system. The latter is not unfrequently turnished with hair-like prolongations, which not only serve to fix the plant, but appear to be much concerned in the absorption of its aliment ; being so much developed in some Lichens, which are located upon the ground, as almost to resemble roots. — In the Fungi we find the same evolution of the more special organ from the more general type. The lower forms of this group (§ 26) seem to imbibe their aliment by their whole surface ; but in the more complex structures, in which the repro- ductive system is separated from the nutritive by the intervention of a stalk (as in the Mushroom), the mycelium at its base is prolonged into filaments, whereby the decaying matter, that constitutes the food of this remarkable group of plants, is introduced into the system. In some species, too, the whole surface is covered with hair, which may assist their very rapid development by absorption of fluid from the atmosphere. In the Mosses and their allies (§ 27), we find a somewhat higher form of the same structure. From the base of the stem there usually proceed slender radical filaments, which sometimes ramify through the soil to a considerable extent ; and other similar filaments are frequently developed from the sides of the stalk, and from the lower surfaces of the leaves. In Mosses that exist on rocks, however, these filaments are but little deve- loped, and appear to serve rather for mechanical support, than for ab- sorption of nourishment, which must in such circumstances be derived from the atmosphere through the leaves. These, as is well known, are o I £11 I 194 OF ABSORPTION AND IMBIBITION. will Mosses to recover the appearance 01 lite alter being long dried ; and the same property enables these beautiful little plants to vegetate rapidly during a moist season, whilst their tenacity of life enables them to with- stand a subsequent drought. — In ascending through these tribes of Cryp- to o'amia, then, we may trace a gradual development of separate absorbent organs, and may observe the specialization of the function, by its restric- tion to one particular part of the surface, instead of being diffused over the whole. The absorbent filaments, however, are very inferior in their structure to true 6 roots,' being little else than elongated cells, resembling the hairs with which other parts of the surface are covered ; and they seem to absorb fluids equally throughout their entire length. Further, we find that, when these special organs are not developed, or are insufficiently supplied with nutriment, the general surface can take-on its original function, and thus supply the deficiency. § de- scending axis' of growth, from w^hich the absorbent fibres are given off; and this is evolved in its completest form in the Phanerogamia (§ 29). In these Vascular plants, moreover, it seems to be through the newly- formed succulent extremities alone, that fluid is admitted; and the func- tion is, of course, more actively performed by them, in proportion to the diminution in the amount of surface they expose. The root presents a great variety of forms in different plants ; there are, however, some parts which are essential, and others that are merely accessory. The simplest form, as well as the most essential part, consists of single fibres ; these occa- sionally exist alone (as at the base of ' bulbs'), but more often proceed from ramifying branches of woody texture (as in most trees and shrubs), or from i tubers' (as that of the turnip). Each radical fibre is of a struc- ture far more complex than the absorbent filaments just mentioned as existing in Cellular plants ; for it is a cylinder, in whose axis lies a bundle of fibro-vascular tissue, whilst its exterior is composed of firm cellular parenchyma; at its free termination, however, the extremities of the vessels are covered with loosely-formed cellular tissue, through which the fluid passes into them. The spongiole, as this point has been termed, is sometimes spoken-of as a distinct organ ; but it is nothing more than the growing point of the root, which, with a few exceptions, lengthens only by additions to its extremity. The soft lax texture of the newly-formed part causes it to possess, in an eminent degree, the power of absorption ; but as the fibre continues to grow, and additional tissue is formed at its extremity, that which was formerly the spongiole becomes consolidated into the general structure of the root, and loses almost entirely its pecu- liar properties. That it is to the spongioles that the principal absorbing power of the root is due, was fully proved by the experiment of Senebier. Having fixed two roots in such a manner, that the extremity of one was in contact with water, whilst of the other every part was immersed except the extremity, he found that the first root absorbed nearly as much as usual whilst the second scarcely took up a sensible quantity. It is not improbable that the relative absorbent power of the spongioles and of the general surface of the root, may vary in different plants, according to the character of the texture of each, and the situation in which it grows ; but it appears to be a general fact, that, in Vascular plants, the spongioles ± i Twf I ABSORPTION IN VASCULAR PLANTS. 195 are the organs specially destined for introducing the fluid nutriment into the syst em. feet long. 175. There are evident limits to the supply of alimentary materials to the roots of Plants, so long as they remain in the same spot ; and some change must take place to ensure its continuance. As the Plant cannot remove itself to a new situation, its wants are provided-for by the simple elongation of its radical fibres ; and their extension takes place, not by increase throughout their whole length, but by addition of fresh tissue to their points. This addition, being made in the direction of least resist- ance, enables the fibrils to insinuate themselves into the firmest soil, and even to overcome the obstacle presented by solid masonry; for however narrow the crevice may be into which the filament enters, the subsequent expansion of the tissue by the infiltration of fluid is so great, as to enlarge the opening considerably, and even to rupture masses of stone. This tendency to increase in the direction of least resistance, will also evidently cause the root to grow towards a moist situation ; and by keeping this in view, many of the facts regarding the so-called instinct of plants, which at first sight appear so remarkable, may be satisfactorily explained. Thus, it was noticed, when the water of the New River was conveyed through wooden pipes, that if these pipes were carried within thirty yards of trees, they were very likely to be in time obstructed by their roots ; which 6 found' the j oints, and then spread out in c foxtails' of fibres, two or three It is well known to the agriculturist, that the course of large drains, even at a considerable depth in the ground, is liable to be inter- rupted by the extension of roots, not only from trees, but also from apparently insignificant plants. Thus at Saucethorpe, in Lincolnshire, a drain nine feet deep was filled-up by the roots of an elm-tree, which was growing at upwards of fifty yards from the drain : a deep drain outside the garden-wall at Welbeck was entirely stopped by the roots of some horse-radish plants, which grew seven feet into the ground : and at lhoresby Park, a drain fourteen feet deep was entirely stopped by the roots of gorse growing at a distance of six feet from it. 46 " — In other cases, we must attribute the result to the dispersion of vapour through the atmosphere in a particular direction. Thus, in a case which fell under the Author's cognizance, a lime-tree, which grew at the distance of about fifteen feet from the shaft of a well, sent a single long root through the soil, in a direct line towards a point of the shaft at which there was a small aperture left by the deficiency of a brick : this aperture was at a height of eleven feet above the usual level of the water in the well ; and the root, having passed through it, divided into a brush-like mass of fibres which descended into the water, and formed a large mass in the lower part of the well. Again, in a peculiar case known to the Author in which one tree grew upon the trunk of another, having originated from a seed deposited at about twelve feet above the ground, one of the lar«-e roots which it sent down, subdivided about two feet above the surface of the ground, instead of proceeding directly down to it, as did all the rest. Now this subdivision took place above a large stone, on the centre of which the root would have impinged, if it had continued to grow directly downwards; and it would appear as if its division, half proceeding to one * " Journal of the Royal Agricultural Society," Vol. I. p. 364. i o 2 I % 196 OF ABSORPTION AND IMBIBITION. side, and half to the other, was due to the direction given to its growth by the ascent of vapour from the soil beneath. On the same principle we are probably to explain the following case. " Near the waterfall at the head of the river Leven, in the Western Highlands, is the trunk of a decayed oak, rotten within, but alive on some parts of the outside. From around protruded reached the ground (a bare rock), runs along the rock in a horizontal position, about thirty feet further, till it reaches a bank of earth in which it has embedded itself." * 176. The absorbent power of the Spongioles appears limited by the size of their pores; for if the roots be immersed in coloured solutions, they take up the most finely divided particles, leaving behind the larger molecules, which are only absorbed when the spongioles have been damaged. The pores are liable to be blocked-up by fluids which are of too viscid or glutinous a consistence to pass readily through them ; and if the roots be immersed in a thin solution of gum or sugar or neutral salts, the watery particles are absorbed in the greatest degree, so that the portion which is left contains a larger proportion of the ingredient in solution. The power of selection, however, would seem to extend beyond this: since of two substances equally dissolved, some plants will take one, and some the other; whilst some neutral salts are rejected altogether. It does not appear that the selecting power is employed to prevent matter, which is capable of exerting a deleterious influence upon the plant, from being introduced into its tissue; for many substances are taken-up by the roots, which speedily put a stop to vital action, if opportunity be not afforded for their excretion. From the little that is at present known on the subject, it seems a reasonable inference, that the rejection of any par- ticular ingredient of the fluid in contact with the roots, results either from an organic change effected by it on their delicate tissue (such as is proved by the experiments of M. Payen t to occur when tannin enters into the solution, even in very minute proportion), or from the want of molecular attraction between its particles and the substance of the spongioles. That some kind of relation between the living tissue and the crystalline cha- racter of the salt, is concerned in the selection, appears from the interest- ing results of the experiments of Dr. Daubeny on the absorption of mineral substances by Plants. He has found that, if a Plant naturally absorbs the compounds of any particular base, it may also take-up those of another base which are isomorphous with them (most vegetables, for example, absorbing the salts of Lime and Magnesia with eaual readiness"): whilst salts, however soluble, which have different from theirs (such as those of Strontia), are not absorbed. X — The a crystalline arrangement fi vary * " Gardeners' Magazine," Oct. 1, 1837. f " Annalesdes Sciences Naturelles," Deuxieme Serie, Botan., Tom. III. p. 5, &c. J "Linnaean Transactions," 1833. — Some recent experiments, however, by the same dis- tinguished Chemist, indicate that potash and soda cannot be substituted, one for the other, in the vegetable organism, to any great extent ; for the proportions of these two bases in the alkaline ash of barley are nearly the same, whether the soil in which the barley is grown be manured with potash or with soda, or be left without artificial addition. (See " Journal of the Chemical Society," Vol. V. p. 9.) ABSORPTION IN VASCULAR PLANTS 197 in the same plant at different periods of the year, and even of the day. The former seems intimately connected with the activity with which the other processes of vegetation are being carried on, and especially to depend upon the quantity of vapour transpired from the leaves (chap. Vii.) ; all the causes which increase exhalation, may therefore be con- sidered as stimulants to absorption also. The vis a tergo possessed by the ascending sap, is sufficiently proved by the celebrated experiments of Hales on the vine. By gages affixed to the stem during the ' bleeding- season,' when the sap rises rapidly, he found that a column of mercury 2 6 inches high, equal to a column of water of nearly 3 1 feet, might be sup- ported by the propellent force of the absorbent organs ; but if the upper part of the plant was cut off, this power soon diminished, and after a time ceased altogether. 177 There would seem much reason to believe, that the mere act ot Absorption in this and other cases, is due to the physical property alreadv referred-to, as possessed by many organised tissues,— viz . the extremities of the spongioles serve as the medium required for this process ; but it may be reasonably enquired whence the other condition is furnished, namely, that difference in density of the fluids on the opposite sides of the septum, which is necessary for the commencement and continuance of the action. This is supplied, in the first instance, by the store of nutritious matter obtained by the embryo from its parent, and contained within its tissues ; and, possibly, at a later period, when the plant is sup- porting an independent existence, by the admixture of a portion of the dense elaborated sap, with the crude and watery ascending fluid. If this be the true explanation of the phenomenon, a counter-current ought to exist, and an exosmose of the fluids within the system should take place into the surrounding medium. That this is actually the case, would appear from the fact, that an excretion of the peculiar products of the species may be often detected around the roots of the plant. The cessa- tion of this action of admixture (a change evidently depending upon other vital actions) at the death of a plant, fully accounts for the non- continuance of endosmose ; which is also checked if the superincumbent column of fluid be not drawn off by the leaves. It has been very justly remarked by Professor Henslow, that, " if we suppose the plant capable of removing the imbibed fluid as fast as it is absorbed by the spongioles, then we may imagine the possibility of a supply being kept up by the mere hygroscopic property of the tissue ; much in the same way as the capillary action of the wick in a candle maintains a constant supply of wax to the flame by which it is consumed."* ^ And this is probably the explanation of the fact, that absorption of fluid continues to take place into the open mouths of the vessels, when the upper part of a plant is cut off, and the divided extremity is immersed in water ; for, so long as exhalation takes place from the leaves, so long will a demand for fluid be created in the vessels from which they draw their supply. 178. It is an axiom in Vegetable Physiology, which has been laid down by De Candolle, " that when a particular function cannot, accord- ing to a given system of structure, be sufficiently carried into effect by * Treatise on 'Botany' in the " Cabinet Cyclopedia, " p. 177. I ; 111 l! II I 198 OP ABSORPTION AND IMBIBITION. * the organ which is ordinarily appropriated to it, it is performed wholly or in part by another." This is a single case of the general principle which has been already laid down (§ 110); and the reason that it is more evident in the Vegetable than in the Animal kingdom, is simply, that in the former the specialization of function is nowhere carried so far as in the latter ; so that any part of the general surface of a plant can perform in a considerable degree all the functions of all the rest. We might then d, priori expect, that whilst the roots are, in the usual condition of the perfect plant, the organs by which its fluid nutriment is absorbed, and the leaves its organs of transpiration and respiration, some traces of the primitive community of function enjoyed by the general surface of the simpler tribes, would be found in the capacity of each of these organs to perform in a certain degree, if required, the function of the other. Thus, it is evident that when the roots are either absent or imperfect, or are implanted in an arid or barren soil, serving merely to fix the stem (as happens with many Orchidece and the gene- rality of aerial parasites), the plant must derive its chief supply of nutri- ment through the absorption performed by the leaves, or, in leafless plants, (as the Gactece), through the general surface. And it must be obvious to all who have observed the manner in which plants, faded by the intense action of light and heat, are refreshed by the natural or artificial application of moisture, that absorption takes place, in these instances also, by the general surface, as well as by the roots. — Various experi- ments have been devised, with the view of determining the relative extent to which the plant is supplied by these two channels ; but the proportion appears to depend upon 1 Mercurialis rowth roots of part of them in water, he placed others so that only their leaves touched the fluid. A small shoot of each plant was kept from contact with water ; and after the experiment had proceeded for five or six weeks, those which had derived all their nutriment through the leaves were nearly as vigorous as those which had imbibed it by the roots. It is by the under surface of the leaf, where the cuticle and cellular tissue beneath it are least compactly arranged, that absorption is performed with the greatest rapidity j and the downy hairs with which some plants are plen- tifully furnished, seem to contribute to this function, acting like so many rootlets. These prolongations of the surface are usually wanting in such plants as grow in damp shady situations, where moisture already exists in abundance ; but in hot, dry, exposed localities, where it is necessary that the plant should avail itself of every means of collecting its food, we find the leaves thickly set with them ; and this diversity may be observed in different individuals of the same species of plant, according to the soil and climate in which they exist, and even in the same individual if transplanted. 179. In tracing the gradual evolution of the special Absorbent appa- ratus of the more perfect Plants, we may observe many interesting relations between the progressive stages of its development, and the per- manent forms of the same system in the lower orders. mbry at its first appearance within the ovule (chap, xi.) is nothing but a single cell, like that of the Protococcus, in the midst of the store of semifluid nutriment prepared by its parent, which it gradually absorbs by its whole ABSORPTION IN ANIMALS. 199 surface, just as do the simplest Cellular plants. At the time of the ripen- ing of the seed, we find a rudiment of the future root, which is deve- loped during germination ; hut in the early stages of this process, tne radicle simply prolongs itself into the ground, and appears to be equally capable of imbibing moisture through its whole length, like that oi the ^ Liverworts or Mosses. It is not until the true leaves are evolved, that the root begins to extend itself by ramification ; then first protruding perfect fibrils, composed of woody fibre and vessels, and terminated by spongioles.— Thus, then, in the development of the Absorbent system oi Vegetables, the first which we have been called upon to study m detail, ■we find a characteristic example of the laws which have been already enunciated (chaps, l, il); for it has been shown that whether we trace its various forms through the ascending scale of the different tribes oi Hants, or watch the progress of its evolution m the more perfect orders, it is constantly to be observed that the speoml structure and function arise by a gradual change out of one more general; and that even where the special ^organ is most highly developed, the general structure , r* ;ams in some degree, the primitive community of function which originally characterised it. 3. Absorption in Animals. 180. It has been shown in the preceding chapter, that the conditions under which the function of Absorption is performed m Animals, are so far different from those which obtain in Plants, that a preparatory pro- cess of Digestion becomes necessary in the former, for the reduction oi the food to the fluid form required for its entrance into the system. 1 his process is effected in cavities of the body, which are bounded by a con- tinuation of its external surface, modified, by its secreting P<> wer > *° supply the means necessary for the solution of the aliment, and, by its absorbent faculty, for the selection of the part of it capable of contributing to the nutrition of the fabric. But so long as this aliment remains un- absorbed, it cannot be regarded as introduced into the system ; since it merely holds the same relation to the absorbent vessels, that the nutri- tious fluid in which the roots of plants may be immersed, bears to the ducts which they enclose. This is brought into clear view by the remark- able fact, that the poison of the most venomous Serpents, and the W oorara- poison of the South American Indians, of either of which a very small quantity will produce death when it is introduced by the minutest punc- ture or scratch into the current of the circulating fluid, are perfectly innocuous when taken into the stomach ; the mucous membrane of the alimentary canal apparently having a peculiar inaptitude for allowing them to penetrate by imbibition, either into the blood-vessels or into the absorbents. Some experiments recently made upon the latter substance, MM. Bernard and Pelouze § o> as confirming substances, however favourable their condition may appear, may be pre- vented by the simply-physical conditions of the membrane which they have to traverse.* * That the absence of poisonous effects from the Woorara poison, when it is simply introduced into the stomach, does not arise from any modification effected in its properties by the agency of the gastric juice, is shown by the fact that the poison, after digestion in 1 ± 200 OF ABSORPTION AND IMBIBITION. 181. It has further been shown that the introduction of the nutritive material into the system, is effected in the lowest animals by simple imbibition into the tissues that surround the digestive cavity, and by " percolation through them towards the more remote parts ; and where such is the case, the digestive cavity either itself occupies a very large part of the body, as in the Hydroid Polypes (§ 1 5 1 ), or prolongations of it, in the form of canals, extend to the parts remote from the principal cavity, as in many of the Acalephse (§153). In most other Invert ebrata, the nutritive materials are taken-up, not directly from the digestive sac, but from the visceral cavity in which it lies. Into this visceral cavity they freely pass, by the apertures that remain patent in the Actiniform and Alcyonian polypes (§ 152); but in all the higher forms of the digestive apparatus, the passage takes place only by transudation through the walls of the stomach and intestinal tube; the chyle, or incipient blood being thus filtered-off (so to speak) from the chyme, or primary product of digestion. In the lowest Mollusca (Fig. 49) as in Rotifera (Fig. 96), and certain Crustaceans (Fig. 105), the flux and reflux of this chylous fluid through the body constitute the only means by which its tissues are supplied with nutriment ; and even in the higher Mollusca, Insects, and Crustacea, the sanguiferous system is in such free communication with the visceral cavity, that their blood cannot be differentiated from its contents. There is in these animals, therefore, no other special absorption, than that which takes place through the walls of the alimen- tary canal. But in the Echinodermata and Annelida, which have a closed sanguiferous system, the contents of this must be taken-up, either from the visceral cavity (which still appears to be the principal channel for the transmission of nutritive materials through the body, their san- guiferous circulation being in all probability chiefly subservient to respira- tion) or directly from the alimentary canal. That absorption does take place m this latter mode, would seem probable from the very minute distribution of blood-vessels upon the surface of the intestinal that fluid for 24 or 48 hours, remains as virulent as ever; whilst the gastric fluid to-which Woorara has been added, loses none of its solvent power. The various secretions which make-up the intestinal juices, have been experimented-on with the same results. Hence it appears that the cause of the innocuousness of the poison, under these circumstances must be looked-for in the gastro-intestinal mucous membrane, which will not give passage to the active principle of the poison, soluble as this is. Experiment proves this to be the case. If the gastric mucous membrane of a recently-killed animal be adapted to an endos- mometer, so that the mucous surface looks outward, and the endosmometer containin g sugared water is then placed in a watery solution of woorara, endosmose will have been found to have taken place in three or four hours, for the liquid will have risen in the tube • and yet this will contain no trace of woorara, as may be ascertained by inoculating with it. If the experiment were allowed to go on for a much longer time, the endosmosis of the poison might occur ; but we should then find that the mucous membrane had under- gone modification, the mucus and epithelium covering it being altered, so that imbibition and endosmosis of the woorara becomes possible ; and if, in place of taking a quite fresh mucous membrane, we take one that has undergone some change, the endosmosis of the poisonous fluid occurs instantly. — As it was interesting to ascertain whether other mucous membranes possessed this resisting power, those of the bladder, nasal fossae, and eyes were tried, and constantly with the same results. An injection was retained in the bladder without in- convenience, for from six to eight hours, by a dog ; but the urine it passed after this time had all the toxical properties of woorara. One mucous membrane alone, the pulmo- nary, is excepted from this immunity ; for the poison, when applied to it, produces the same effects as when introduced into the sn^nt.QTmmnc. or-^ior fi^ano — "t.'tt™,™ \kzai cale," 1850, No. 125. subcutaneous areolar tissue. L' Union Medi- ABSORBENT SYSTEM IN ANIMALS. 201 Fig. 108. -?/.!/' lit tube, which will be shown to exist in the Holothuria (Fig. 40) and m many Annelida (Figs. 114, 115). 182. In the Vertebrata, however, it is by vessels alone, that the nutri- tive fluid is removed from the alimentary canal, the Avails of which do not allow it to transude into the surrounding cavity. And we here find provided, in addition to the blood-vessels that are copiously distributed upon the coats of the gastro-intestinal tube, a special set of Absorbent vessels, ^hich seem to be destined, not only for the introduction of nutritive mate- rials into the system, but also for submitting these to a certain preparation (chap, viii.), before they are admitted into the current of the circulation. The ' Absorbent system' of vessels consists of two principal divisions, ^hich may be compared to two sets of roots proceeding from a common trunk j one of these commences upon the walls of the intestines, and is termed the < Lacteal' system, from the milky character of the < chyle' which it contains ; whilst the other takes its origin in various parts of the substance of the organism at large, especially in the skin and sub- cutaneous textures, and is known as the < Lymphatic' system, from the transparent watery aspect of the liquid it conveys.— Although the walls ? f the whole gastro-intestinal canal are furnished with Absorbent vessels, in common with other membranous surfaces, yet it is on those of small intestine, below the point at which the liver and pancreas dis- charge their secretions, that the Laeteals espe- cially abound; and they seem, in the higher Vertebrata at least, most commonly to origi- nate in the interior of the villi, where they are surrounded by the plexus of capillary Wood- vessels that lies immediately beneath the external surface of these filamentous pro- cesses (Fig. 108). In Fishes, however, the villi are few, or are altogether absent, and the cteal trunks receive their supplies through a coarse plexus situated in the walls of the intestinal canal. Such a plexus is seen also in Reptiles, in which villi are developed ; and it seems probable that it exists in the higher Vertebrata, in which the mucous membrane is much more thickly set with villi, and in which it appears to be chiefly through the laeteals contained in these, that the chyle gains admission into the larger trunks. The precise mode in which the laeteals commence near the free extremities of the villi, cannot be stated with certainty; but it is probable that they form loops by anastomosis with each other, so that there is no proper free extremity in any case. It is beyond all doubt, how- ever, that the laeteals never commence by orifices upon the internal surface of the intestine, as was formerly imagined. When these vessels are turgid with chyle, the extremity of each appears to be imbedded in a collection of globules, presenting an opalescent appearance, which give to the end of the villus a mulberry-like form; and this appearance is due to the distention of the epithelial cells covering the extremities of the villi, with the oleaginous chyle which they have absorbed, and which they .umi \\ : ! J 1 « i Ma L ^* Li 202 OP ABSOBPTION AND IMBIBITION. afterwards yield-up to the lacteals, returning to their original condition when this selective operation has been accomplished.* 183. The Lymphatic vessels are distributed in the greater number of tissues and organs which possess vessels for the conveyance of blood; but to this general statement, there are some remarkable exceptions ; for they are entirely wanting in the substance of the brain and spinal cord, although they are found in their investing membranes ; and they occur very scantily m the muscles. It appears to be in the skin and the sub- cutaneous textures, at least in Man, that they are most plentifully distri- buted; and they seem there to originate, like the lacteals of the intestinal walls, m plexuses of which the meshes are very close. According to Prof. Kolliker, the lymphatics in the tail of the Tadpole do not form a network, but branch out like rootlets, their ultimate extremities, or rather their commencing radicles, having free but closed ends, runnino- out into fine points ;t it may be doubted, however, whether this is not the result of a want of completeness in their development, and whether these ramifications would not meet and inosculate in the fully-developed Frog. § in stellate cells, which send forth long proj ections ; but these branches do not inosculate with each other in any other way than to form con- tinuous tubes; and if they subsequently constitute a plexus, it must be by the development of connecting arches at a later period. — It has been maintained that the minute lymphatics communicate with the capillary vessels in their neighbourhood ; and these communications have been supposed by some to allow the direct transmission of the lymph into the sanguiferous system ; whilst by others it has been inferred that the liquor sanguinis, or fluid portion of the blood, (which, when diluted, closely resembles the contents of the lymphatics in its composition) finds its way into the absorbents. It is nearly certain, however, that no such apertures exist; and that when any direct passage of fluid does take plape from one set of vessels into the other (as is often the case in artificial injections, and was noticed by Prof. Kolliker in watchino- the circulation in the tail of a Tadpole which had been injured), it is through an abnormal opening. 184. The walls of the absorbent vessels lym phatics) are extremely thin, so that the character of their contained fluid can be readily discerned through them. Those which form the ultimate ramifications of the system, appear to be limited only by a very delicate homogeneous membrane ; and it is not certain that even this is univer- sally present in the absorbents of Fishes. A similar membrane, covered with a layer of pavement-epithelium upon its inner or free surface, constitutes the lining of the mid-sized and larger trunks; but these possess, in addition, a middle or fibrous layer, in which non-striated muscular fibres may be distinguished, and an external sheath of areolar * By Prof. Goodsir, who was the first to direct attention to the peculiar appearance presented by the cells at the extremities of the villi during the process of lacteal absorption, it was maintained that the ordinary epithelial cells fall-off, and that the chyliferous cells are developed de novo within the villus, that is, beneath its basement-membrane. The researches of several excellent observers, however, have shown this view to be erroneous, and have established that stated in the text. (See especially Prof. Kolliker' s "Mikro- skopische Anatomie," Band ii. § 169.) + "Annales des Sciences Naturelles, " 3 e Ser., Zool., Tom. VI., p. 98. i * ' ■ ■ ABSOKBENT SYSTEM IN ANIMALS. 203 tissue. furnished valves, which allow the passage of their contents in only one direction, sanguiferous system ; these valves, however, seem to be wanting in the ' plexuses of origin/ which are filled by mercury injected into any part of them. In Fishes and Reptiles, however, in which this system is obviously deve- loped upon an inferior type, the valves are few or are altogether In the higher Yertebrata, again, certain small solid bodies are wanting. found in the course of the lacteals and lymphatics, which are termed 'absorbent glands.' The structure of these, however, does not cor- respond with that of ordinary glands ; and they have been more appro- priately named ' ganglia/ for they are essentially composed of plexuses of absorbent vessels, convoluted (so to speak) into knots, and dilated into larger cavities, amongst which capillary blood-vessels are minutely distributed; the whole being bound together by areolar tissue and invested in a capsule of the same. These blood-vessels have no direct communication with the interior of the lacteals, but are separated from them by the membranous walls of both sets of tubes; so that whatever passage of fluid normally takes place from one set of vessels to the other, must be accomplished by transudation through these.* According to the observations of Prof. Goodsir, the absorbents, when they enter a gland, lay aside all but their internal coat and epithelium; and the latter, in place of retaining its pavement-like character, presents itself as an irregular layer of spherical nucleated corpuscles, measuring about 1 -5000th of an inch in diameter; which layer is thickest in the dilated lymphatics which form the ' cells' of the centre of the gland, and becomes gradually thinner towards the periphery, where it is continuous with the epithelium of the afferent and efferent vessels. The inner layers of the central epithelium appear to have no tenacity; so that the component cells may be readily detached from one another, and carried-off in the fluids which traverse the cavity. — Having thus considered the general structure of the Absorbent system, we shall proceed to notice its more special peculiarities in the different classes of Yertebrata. 185. The proper Absorbent system is exhibited in its simplest and most diffused form in Fishes, the lowest class in which its existence has been demonstrated. Where it consists of distinct vessels, their walls are very thin and distensible. The Lacteals commence in a somewhat coarse network of canals, that seem channelled-out (as it were) beneath the mucous lining of the intestinal canal, and cannot be shown to possess definite walls ; from these, however, the chyle is conveyed away by proper vessels, which form capacious plexuses in the mesentery, along the course of the alimentary canal. The Lymphatics are distributed extensively through both the superficial and the deep-seated parts of the body; and, they also, by the convolutions and anastomoses of their trunks, form numerous plexuses in various situations, especially around the veins, which may be regarded as the first indications of the so-called * It has been asserted by many anatomists, that free apertures exist, by which the contents of one set of vessels can pass directly into the other ; but these statements are founded on the results of injections, which can be very easily forced to make such aper- tures ; and the most careful examination has failed to detect them, when no such procedure has been employed. iilfl I HMM ' "-v — F I H 204 OF ABSORPTION AND IMBIBITION. cavity of many Fishes, externally to the jugular veins. < glands' that are presented in the higher classes. Some of these trunks, moreover, dilate into sinuses, which appear to be contractile, and pre- figure the ' lymphatic hearts' of Reptiles. Such a sinus may be seen in the tail of the Eel; and another is found on each side of the cranial Although a con- siderable proportion of the lymphatic " trunks unite with the lacteal vessels, to form principal canals (corresponding with the thoracic duct in higher animals), which empty their contents into the systemic veins near the heart, there are many other communications between the two systems, as Fohmann appears to have satisfactorily demonstrated. Thus, the caudal sinus of the Eel discharges its contents into the caudal vein, the orifice being provided with a valve. 185. The conformation of the Absorbent system presents several in- teresting peculiarities in the class of Reptiles. As in Fishes, the vessels are generally destitute of valves, though these may occasionally be observed in the larger trunks ; but they everywhere seem to possess distinct walls. When compared with that of Birds and Mammals, the absorbent system of Reptiles seems to possess an enormous extension ; large and capacious lymphatic plexuses being developed around the great veins, and the length of the trunks being often augmented by doublings and convolutions. But this extension is rather apparent than real ; for there is still an absence of the < glands,' which seem to concentrate, as it were, the assimilating power of a long series of tubes ; and the relation of the Absorbent system of Reptiles to the more concentrated apparatus of Birds or Mammals, thus comes to resemble that which the extended tracheal system of the Insect bears to the lungs of the higher Yertebrata. — The Lacteals in Reptiles, as in the classes above them, partly commence in the i villi' of the intestinal canal ; but, as in Fishes, there is also a very coarse plexus of absorbents beneath its mucous lining. The fluid which they absorb is collected into a receptaculum chyli, situated at the root of the mesentery; and from this it passes by two or more ducts, which also receive many of the lymphatic trunks, into the great systemic veins. The Lymphatic portion of the system is furnished, in most Reptiles, with certain pul- sating dilatations, or lymphatic hearts, which aid in the propulsion of the contents of the vessels; the walls of these contractile cavities are formed of striated muscular fibres. In the Frog there are two pairs of them; one situated just under the skin, through which its pulsations are readily seen in the living animal, immediately behind the hip-joint; while the other pair is more deeply seated at the upper part of the chest. The former receive lymph from the posterior part of the body, and pour it into the veins proceeding from the same part, by orifices furnished with valves; the latter collect that which is transmitted from the anterior part of the body and head, and empty their contents in like manner into the jugular vein. Their pulsations are totally inde- pendent of the heart and of the acts of respiration, since they continue after the removal of the former, and for an hour or two after somatic death and the complete dismemberment of the animal. Neither are they synchronous with each other on the two sides of the body, nor always performed in the same space of time; for the pulsations are not only generally irregular, but sometimes exhibit long and frequent inter- missions; when in constant action, they occur about sixty times in a - ABSORBENT SYSTEM IN ANIMALS 205 minute. From the observations of Volkmann, however, it appears that these movements are dependent upon the connection of the lymphatic hearts with the corresponding segments of the spinal cord ; since they cease when these are destroyed, although all other parts may be left uninjured ; while they continue so long as this connection remains perfect, although all other parts of the nervous centres be destroyed.*— A pair of vesicles similar to the posterior pair in the Frog, has been detected m Salamanders, Lizards, and Crocodiles, in which they are situated near the root of the tail, and are connected, in like manner, with the veins of the lower extremity; they have also been discovered in Serpents, where they lie under the last rib. It was for some time believed that the Ckelonia formed an exception to the general fact of the existence of such pulsating receptacles in Reptiles; but these have been shown by Muller to be particularly large in that group, in which they lie behind the superior extremitv of the iliac bones, receiving the lymph by capacious trunks from the posterior extremities, and pouring it into veins that discharge themselves into the reno-portal system, t ... „ , 186 In Birds, we find the Absorbent system existing m a more perfect form • its trunks being everywhere provided with valves, and the diffused plexuses being partly replaced by < glands' or 'ganglia,' which may be probably considered as performing the same function by an organisation of more concentrated character. The lacteals, which are not furnished with these glands, all converge towards a receptaculum chyli, from which proceed two thoracic ducts, one on either side, to terminate in the angles formed by the junction of the jugular and subclavian veins. These ducts receive, also, most of the lymphatic trunks ; but the lymphatic system has two other communications, as in Reptiles, with the veins of the lower extremity. These are connected with two large dilatations of the lymphatic trunks, which are evidently analogous to the lymphat of Reptiles, but which do not seem to have any power of spontaneous movement. In the Goose, they are about the shape and size of a kidney- bean, and are situated in the angle between the tail and the thigh. They were supposed by Panizza to possess an automatic power of alternate r>m+™^'™ o^/i /ma+a+irin • Tvn+. thfisfi motions have been shown by Miiller with them, and to be due to the respiratory actions, ceasing when they are interrupted. 187 In the Absorbent system of Mammalia, we witness its most concentrated and highly-developed form. The vessels are copiously pro- vided with valves ; and their parietes are firmer than m the lower classes. Instead of the extensive plexuses of Fish, we find small dense ' glands ' disposed in different parts of the system ; these are more numerous than in Birds, and present themselves on the lacteals ^^ well as on the lvmnhatics being known in the one case as the ' mesenteric,' and in the other as ' lymphatic' glands. In some Mammalia, especially of the order Garnivora the mesenteric glands cluster together into a single mass, named the pancreas Asellii, which lies at the root of the mesentery. The Lacteals all discharge their contents into the receptaculum chyli, which is situated in the lumbar region, close to the spine ; into the same recep- tacle, many Lymphatic trunks pour the fluid which they have collected * << Muller's Archiv.," 1844. f Ibid. 1840. I I il . f I m _ mm «■ JO 206 OF ABSORPTION AND IMBIBITION. from the posterior part of the trunk and extremities \ and from it arises the Thoracic Duct of the left side, which passes forwards along the spine, receiving other lymphatic trunks in its course, and terminates at the junction of the left jugular and subclavian veins. A smaller trunk on the right side receives the lymphatics of the right side of the head and upper extremity, with those of the right lung and right side of the liver j and this terminates, in like manner, at the junction of the right sub- gul It is a beautiful instance of mechanical adap- tation, that as the angle formed by the convergence of two veins is a point of much less resistance than any other part of the walls of the vessels, the easiest possible entrance is thus provided for the fluid dis- from the thoracic ducts into the current of the circulation. charged Although these are the only two canals by which the Absorbents usually communicate with the veins in Man, their number is greater in many species of Mammalia; they all terminate, however, in the same part of the venous system. Thus, the left thoracic duct often resembles rather a plexus of vessels than a single tube ; branches proceeding from it and then reuniting, and at last terminating in the veins by several apertures. Sometimes it consists throughout of two tubes, which anas- tomose with each other and with the duct on the right side, and terminate separately in the veins ; and in the Pig a branch of communication is sent off to the vena azygos, which is a small trunk running in proximity with it along the spinal column. All these modes of distribution occur w darities of conformation in the Human subject, the former not being uncommon ; the last, however, is rare. 188. The cause of the onward movement of the contents of the Ab- sorbent vessels in the higher Yertebrata which have no i lymphatic hearts,' and also of the flow of fluid towards these pulsating cavities in the animals which possess them (situated, as they are, close to the points where the fluid of the absorbents is discharged into the veins), has not been posi- tively determined. This movement may be partly attributed to the vis a tergo, produced by the continual imbibition of fresh fluid into the rootlets (so to speak) of the vascular tree, and partly to rhythmical contractions (with alternating dilatations) of the villi themselves, as observed by MM. Gruby and Delafond;* and although it may be thought, from the extreme dis- tensibility of the walls of the absorbents, that such forces will be rather expended in dilating them, than in pushing onwards the column of liquid which they contain, yet it must be remembered that they are surrounded by tissues, whose tonicity, during the living state, gives to them much more resisting power than they possess after death. Further, in all the movable parts of the body, assistance is doubtless afforded by the occcasional pressure which will be exercised upon the absorbents by the surrounding tissues; for while this pressure is operating, it will tend to empty them of their contents, which are only permitted by their valves to pass in one direction; and when the pressure is relaxed, they will be re-filled from behind. But it seems probable that the regular propulsion of the fluid mainly depends upon an alternate contraction and dilatation of successive portions of the vessels, slowly repeated at intervals; such alternations having been witnessed by Prof. Kolliker in the tail of the * "Comptes Rendus," 1842, p. 1199, and 1843, p. 1195. ABSORPTION IN ANIMALS. 207 tadpole ; and it being apparently by such contraction, without subsequent dilatation and re-filling, that the absorbents are emptied after death, and this with considerable rapidity. 189. We have now to enquire into the relative parts which are per- formed in the function of Absorption, by the proper Absorbents, and by the Blood-vessels ; and although these cannot yet be said to be precisely definable, yet there can be little doubt that we are in possession of the general truth with regard to them. — From the time when the Lacteal vessels were first discovered, down to a comparatively recent period, it was supposed that they constitute the channel through which all fresh nutritive material is taken into the body; and this idea seemed to derive confirmation from the great uniformity in the composition of the chyle, which, though different as a whole from that of blood, appeared adequate to supply those substances to the latter, which are most constantly being eliminated from it by the nutritive and respiratory operations. Various considerations, however, would lead to the conclusion, that the nutritive materials do not enter through the lacteals alone ; but that the most soluble portions of them, together with other substances not nutritious, find their way directly into the blood-vessels. — In the Invertebrata, it will be recollected, no special Absorbent system exists ; and in all those which possess blood-vessels, it is by them alone that alimentary matters are introduced into the system ; it would not seem likely, therefore, that this most general method of performing the function should be entirely superseded in Vertebrata by the more special one. But further, when the extraordinary vascularity of the whole gastro-intestinal membrane is con- sidered, together with the peculiarity of the special distribution of the capillaries in the villi ; and when it is remembered also that the rapid movement of blood through these, creates the condition most especially favourable to the passage of liquids into them from the outside (§ 172), it might be almost certainly affirmed that endosmose must take place between the contents of the alimentary canal and the blood in the vessels. confirmed Thus MM substances were mingled with the food, which might be easily detected by such as gamboge, madder, colour, odour, or chemical properties, were their camphor, musk, asafoetida, and various saline substances, — they seldom found in the chyle, though many of them were detected in the blood, and some had even passed into the urine. So, again, it was found that if any of these substances be introduced into a portion of the intestine separated by ligatures from the remainder, and all the vessels of that portion be divided or tied, save its artery and vein, the substance may speedily be detected in the blood, and if it be poisonous its effects are manifested nearly as soon as usual ; whilst, if the blood-vessels be tied, the lacteals being left entire and uninterrupted, a long period elapses before there is any evidence of absorption. It cannot be doubted, then, that alimentary substances in a state of solution, such as albumen, gelatine, or sugar, may pass into the blood-vessels by simple endosmose, when the relative densities of the blood and of the intestinal liquids are such as to favour the inward current. Conversely, it might be inferred that if the liquid in the intestines be of a nature to determine the endos- motic current in the contrary direction, some of the constituents of the II : i i II I 208 OF ABSORPTION AND IMBIBITION. blood would be drawn from them into the alimentary canal. Now it has been shown by the experiments of Poisseuille, that an endosmotic current takes place through animal membranes from the serum of the blood towards certain saline solutions ; and thus it happens that when these are taken into the alimentary canal, they produce a copious exudation of fluid from its walls, which fluid contains a considerable quantity of albumen.— It appears, then, that an interchange between the contents of the blood-vessels and those of the alimentary canal takes place, in either direction, in a manner which is in all respects conformable to physical principles ; so that Absorption must be effected through the blood-vessels of the higher animals, as of the lower. 190. On the other hand, the Lacteals, as already pointed out, would appear to receive only substances of a particular class, more especially fatty matters in a state of more or less fine division, with which albumi- nous compounds are intimately mixed. These, being first drawn-in by the epithelial cells at the extremities of the villi (§ 1 82), and then trans- ferred to the lacteals, are doubtless obtained directly from the food; for the quality of the Chyle depends upon that of the aliment last digested, it being of an opaque white if that food contained much oily or fatty matter, and of a more transparent aspect if such matters were deficient. Moreover, the experiments of Bouchardat and Sandras* have shown, that particular kinds of fatty substances with which animals may have been fed, are recognisable in the chyle ; and on the whole it may be considered, that the special function of the lacteals is to take up the oleaginous portion of the food, and to bring it into that intimate relation with albumen which seems to be requisite for its subsequent assimilation (chap, viii.) Still it appears by no means certain that the blood-vessels also may not absorb oleaginous substances, as they do in Invertebrated animals ; particularly as it has been shown by the experi- ments of Prof. Matteucci,t that if an emulsion be made by shakino- a few drops of oil in water to which a little alkali has been added, and an endosmometer filled with a weak alkaline solution be then immersed in this emulsion, the oil penetrates in a short time through the membranous partition, and makes its appearance in the interior of the endosmometer Now as the blood is slightly alkaline, and as its density favours the imbibition of fluid, there seems no reason why fatty matters should not thus find their way into the blood-vessels, when they have been reduced to a state of fine division in the alimentary canal, and have been rendered alkaline by the admixture of the biliary and pancreatic fluids. 191. With regard to the relative share taken by the Lymphatics and the Blood-vessels, in the absorption of fluids by the external surface or from the closed cavities of the body, and in that ' interstitial' absorption which removes the solid particles of the fabric when they no longer retain their vital endowments, there has been a yet greater amount" of misapprehension. It was long imagined (the doctrine having been strongly sustained by John Hunter and his immediate followers), that the office of the Lymphatic system is to take-up and remove all the effete matter which is to be cast out of the body, as no longer adapted to form part of * " Annales des Sciences Naturelles," 2° Ser., Zool., Tom. XVIII., XX. t " Lectures on the Physical Phenomena of Life," Dr. Pereira's Translation, p. 111. ** ft m^^m LYMPHATIC ABSORPTION IN ANIMALS. 209 it, and as inconvertible into any other useful product. I For such an idea, however, there is not the least adequate foundation. The liquid contained in the lymphatics has all the characters of dilute c liquor sanguinis' (the liquid portion of the blood in which the corpuscles float); &nd it differs from that of the lacteals chiefly in the absence of fat. The lacteals, indeed, when the alimentary canal is empty, seem to perform the function of lymphatics; being found to contain a fluid which resembles lymph in every respect, and is probably derived from the same source. Again, as the lymphatics do not discharge their contents into any of the outlets through which they might be carried off, or convey them to the excretory glands by which they might be eliminated under some other form, but pour them into the same receptacle with the nutrient materials newly imbibed from the food, whence both are propelled together into the general current of the circulation, it seems almost certain that their contents, in whatever mode obtained, are destined to be employed in the formation of the tissues, and not' to be forthwith eliminated from the system. 192. With respect to the source of the Lymph, and the manner in "which it is imbibed, there is at present a deficiency of accurate know- ledge. It is very probable, however, that it partly consists of the residual fluid, which, having escaped from the blood-vessels into the tissues, has furnished the latter vdth the materials of their nutrition, and is now to be returned to the current of the circulation. But it also seems not unlikely, that it may partly be derived from those particles of the solid framework, which have lost their vital powers, and are therefore unfit to be retained as components of the living system, but which have not undergone such a degree of decay, as to prevent them from serving, like the aliment derived from the dead bodies of other animals, as a material for reconstruction, when it has been again subjected to the assimilating process.* In what way the selective power is exercised, by which extra- neous substances are usually prevented from entering the lymphatic system, when they are quickly received into the blood-vessels, cannot at present be even guessed-at : since there is no reason to believe that the lymphatic l plexuses of origin' are in relation with cells (like those of the villi) to which such a function might be attributed.— is not so constant for the lymphatics as for the lacteals, yet it does not appear that these absorbents readily take up liquids in contact with the skin, unless they be of an alimentary character, assimilating in com- position to lymph. There are certain saline compounds, however, which seem to pass as readily into the lymphatics, when applied in solution to the cutaneous surface, as into the blood-vessels, or even more readily; no general rule, however, can be laid-down upon this subject; and it is pro- bable that differences in the relative distribution of the two orders of vessels in the skins of different animals, may considerably influence the result. A peculiar aptitude of the lymphatics for the absorption of * In this point of view, the almost entire absence of lymphatics from the Muscular and Nervous tissues presents an obvious signification ; since when these tissues are disinte- grated by being called into vital activity, their elements at once pass into new states of combination, which, being purely excrementitious in their character, are not fit to be received into the lymphatic system for the purposes of nutrition, but are directly conveyed by the sanguiferous current to the organs which are charged with their elimination from Although the rule the body. a >■ I « ft ; P T*- i 210 OF ABSORPTION AND IMBIBITION. milk seems to be shown by the experiments of Schreger, who found that the lymphatics of a limb long immersed in it became turgid with this fluid : that none of it could be detected, however, in blood drawn from the part, was sufficiently accounted-for by the circumstance that a bandage had been tied round the limb, thereby producing turgescence of the superficial veins. The fact that the lymphatics in the neighbourhood of collections of peculiar animal fluids, sometimes become filled with those fluids, — as bile from an over-distended gall-bladder, or pus from an abscess, need not be regarded as invalidating the general statement already made with regard to their probable function; for there can be no doubt that under such circumstances a direct entrance might be readily gained into the absorbents, either through rupture or ulceration of their walls ; and in this way alone could particles so large as pus-globules find their way into these vessels. The case is different, however, with regard to substances introduced through the skin by friction ; for these often seem to find their way quickly into the lymphatics, as is shown by the circumstance that if they be of an irritating character, red streaks appear along the course of the absorbents, and the neighbouring glands are swollen. This is probably to be explained by the peculiarly abundant distribution of the lymphatics in the skin, and the ready access which liquids can obtain to their walls. 193. There can be no reasonable doubt that it is by the Blood-vessels, rather than by the Lymphatics, that all that c interstitial ' and c super- ficial' absorption is performed, which is not of a directly nutritive character ; and in particular that those effete matters are carried away from the tissues, which are destined to immediate elimination. Thus, the most rapid and constant in its formation of all the excretory products, and the most injurious if retained, is carbonic acid; and this is conveyed by the venous system to the Lungs, in which it is forthwith removed from the blood. So again, it is by a special arrangement of a part of the venous system, that the Liver is supplied with venous blood for the elaboration of bile ; indicating that it is in such blood that the elements of the bile most abound. And this will be seen to be the case with regard to the Kidneys also, in the lower Yertebrata; although in the higher, it is from the arterial system that they receive their supply of blood for secretion as well as for nutrition. Further, as no lymphatics exist in the whole Invertebrated series, it is obvious that nothing but the sanguiferous system can perform the entire function of interstitial absorption ; and the same must be the case in those tissues of Vertebrata, which are 'destitute of special absorbents. Experiments demonstrating the exercise of the absorbent power by the blood-vessels, upon substances placed in contact with them, are most numerous and convincing j it will be sufficient to mention two. Mayer, having injected a solution of prussiate of potash into the lungs, detected it in the left cavities of the heart sooner than in the right ; whence it is obvious that it must have been absorbed by the pulmonary blood-vessels more speedily than by the lymphatics of the lungs. And when the jugular vein of a young dog was laid bare by Magendie, and a solution of mix vomica was supplied to its external surface, the symptoms of poisoning manifested themselves in four minutes. 194. It may be stated, then, as a general fact, that the Blood-vessels ^M^BM -T ABSORPTION BY GENERAL SURFACE IN ANIMALS. 211 are the principal channels, through which water and substances dissolved in it are introduced into the body, either from the walls of the alimentary canal, or from the general surface; and by which that interstitial absorption is effected, whereby the particles that have served their pur- pose in the solid fabric are removed from it: — But that the Absorbent system, possessed by the higher animals, is the special channel for the introduction of oleaginous matters, suspended in an albuminous fluid, from the intestinal tube ; and for the return to the sanguineous circu- lation of such matters as may be yielded-back by the tissues (whether from the superfluity imparted to them by the blood, or as products of their own disintegration), in a state to be again employed for the purposes of nutrition. We shall hereafter find reason to believe (chap, ix.), that during its slow movement through the absorbents, and especially during its passage through their glandulse, the Chyle and Lymph undergo changes by which it is brought into a nearer likeness to the Blood, more especially in regard to the vital properties of that fluid. 195. Notwithstanding that it is through the walls of the alimentary canal, in most of the higher animals, that the introduction of nutriment from without is chiefly effected, yet it must not be lost sight of that the general surface of Animals, as of Plants, is still to a certain degree capable of taking this function upon itself; and this not merely when the regular channels have been closed, but even in some cases as the regular func- tional duty of that part. Thus the experiments of Dr. Madden* show, that a positive increase usually takes place in the weight of a man immersed in a warm-bath, even though there be at the same time a loss of weight by pulmonary exhalation and by transudation through the skin ; and he found that when this loss was taken into account, the quantity of water absorbed was about an ounce and a half in half an hour. The absorption will be more rapid, when the fluids of the body have been previously diminished by unusual exhalation; thus Dr. S. Smith mentions that a man who had lost nearly three pounds by perspiration during an hour and a quarter's labour in a very hot dry atmosphere, regained eight ounces by immersion m a warm bath for half an hour. And a patient into whose stomach no aliment of any kind could be introduced, has been kept alive for some time chiefly by cutaneous absorption, the body having been immersed night and morning in a bath of milk and water, and imbibing from 24 to 36 ounces per diem. — Even the vapour of the atmosphere may, in certain cases, afford the requisite supply, drink Thus Frogs seldom or never as a but they habitually live in a moist atmosphere ; and when they have lost fluid by exposure to hot dry air, they will regain their weight by being left for a time upon moist sand, and the bladder (which serves reservoir of water for cutaneous exhalation) though previously emptied, will be refilled, t It is probable that the same may occur in all animals with a soft naked skin; and there are cases which (if the facts be correctly reported) would seem to prove unequivocally, that the cuta- neous and pulmonary surfaces even in Man may serve upon occasion for the introduction of large quantities of fluid from the vapour of the atmo- sphere, f * t + " Prize Essay on Cutaneous Absorption," pp. 59 — 63. See Art. 'Amphibia' in "Cyclop, of Anat. and Physiol.," Vol. i. p. 104. See the Author's " Human Physiology," §§ 468—470, and the cases there cited p 2 i ! 212 OF THE CIRCULATION OF NUTRITIVE FLUID. CHAPTER V. 1 OF THE CIKCULATION OF NUTKITIVE FLUID. 1. General Considerations. 196. In beings of the most simple organisation, whether belonging to the Animal or to the Vegetable kingdom, we have seen that every part of the surface is equally capable of absorbing the liquid aliment brought into contact with it ; and that the materials of the tissues are supplied by the continual imbibition of the nutriment thus immediately derived from external sources. In such, therefore, it might be inferred that no transmission of fluid from one portion to another would be required for the purposes of the economy; and we find no evidence of its existence, either in a structure specially adapted to it, or in any visible motion of such fluid. But as, in more complex organisms, a small part only of the surface is particularly appropriated to the function of Absorption, it becomes evidently necessary that means should exist, for conveying to distant parts the nutriment they require. This is effected by the Cir- culation of the nutritious fluid, through a system of vessels or passages adapted to this purpose; and it may be regarded as a general statement of the condition of this system in all classes of living beings, that its development is proportional to the degree of limitation of the power of absorption, by which the parts directly imbibing aliment are removed from those requiring supplies. — But the conveyance of nutrient fluid to the remote parts of the organism, is not the only object to be fulfilled by the circulating apparatus ; since the crude aliment nrast be exposed to the influence of the air before it becomes fit for its ultimate purpose, and that which has once passed through the tissues of Animals must undergo a similar process to restore it to its proper condition. This process, which constitutes the function of Respiration (chap, vl), requires that the circulating fluid should pass, in all highly-developed organisms, through certain organs specially adapted for its performance ; and hence the arrangement of the circulating system is modified, not only for con- veying the alimentary materials from the part of the system where they are introduced to that where they are required, but also for causing it to be brought, during some part of its transit, into relation with the atmosphere. It is very evident, therefore, that the uninterrupted per- formance of this function is essential to the continuance of life : since not only does the nutrition of the tissues, or ' vegetative life,' wholly depend upon the materials thus supplied ; but the presence of oxygen conveyed by the vital fluid is necessary for the active performance of the 6 animal functions' by the nervo-muscular apparatus (§ 93). 197. In the study of the Circulation, we shall have reason to see the peculiar advantage to be derived from the investigation of the simplest • conditions under which it may be performed. It has been from the confinement of their attention to this function, as it exists in the higher Animals only, that many Physiologists have adopted incorrect and narrow Mi^^Bi S- CIRCULATION IN CELLULAR PLANTS. 213 views as to the powers by which it is maintained ; — views which are inca- pable of extension to the whole Animal kingdom, far less to the Vegetable creation, and which must therefore be fundamentally erroneous. We shall endeavour to show that principles of wider comprehensiveness may be attained, by comparing the principal facts relative to the circulation of nutrient fluid, derived from all the classes of living beings in which it presents itself. 2. Circulation in Vegetables. 198. The tissues of the lower tribes of Cryptogamia, being almost entirely cellular in their structure, do not seem to be adapted for any very regular or definite transmission of fluid. The Algce, as already stated, absorb by their whole surface ; and there appears to be so little com- munication in this class between different parts of the same individual, that, if one portion be suspended out of the water, it will dry up and die, whilst that which remains immersed will preserve its freshness. No trace of vessels is discoverable in this order ; the ^ cells present a rounded form in almost every part ; and the only deviation from this arrangement occurs in the ' veins' which strengthen the foliaceous expan- sions of some of the higher species, in which we find the cells somewhat elongated, and presenting an approach in form to woody fibre. — Amongst the Lichens, a similar uniformity of structure prevails ; no appearance of vessels is perceptible ; but wherever the form of a stem is assumed, the cells, which are rounded in the foliaceous expansions, possess more or less of elongation. As in this tribe the power of absorption is usually restricted to the side least exposed to light, some capability of diffusing the nutrient fluid is required ; and it appears that, when the absorbent surface is placed in water, the liquid is slowly transmitted in the course of the elongated cells, to the whole plant. — In the higher Fungi, we may trace a further development of this simple form of the Circulating apparatus. In those species whose reproductive apparatus is elevated on a stipes (as in the Mushroom tribe), the nutriment, which is entirely received by the mycelium at its base, is transmitted by its elongated cells, and probably through certain hollows left by the separation of the tissue (termed inter-cellular spaces), to the expansion on its summit, where it is diffused in every direction. — It may be regarded, therefore, as a general expression of the function in these Cellular plants, that, when there is no tendency to prolongation in a particular direction, and the cells retain their rounded form, they transmit fluid with equal readiness towards all sides ; but that, when any separation of the different parts takes place, by the restriction of the function of Absorption to one portion of the surface, there is a tendency to the evolution of an axis formed of pro- longed cells, in the direction of which the fluid is conveyed most readily to the other parts of the system ; its course being most rapid in those parts in which the laxity of the tissue and the direction of the cells oppose the smallest amount of resistance, just as we see liquids penetrating easily through unsized paper. 199. In the higher group of Cryptogamia, consisting of the Mosses and Ferns with their allies, we find a much more evident approach to the vascular structure and general circulation of the Phanerogamia. Still, however, its lower tribes (such as the Hepaticce, § 27) are so closely I D i III! I J-- 214 OF THE CIKCULATION OP NUTRITIVE FLUID connected with the more perfect forms of the preceding group, that what has been said of those will be equally applicable to them. Among the Mosses, strictly so called, we find several species in which a complete stem is developed, furnished with radical fibres at its base, and bearing a number of veined leaves regularly arranged upon it. cellular tissue between the In these, the central and cortical portions of the stem and in the mid-veins of the leaves becomes considerably elongated, so as almost to resemble woody and vascular structure ; it does not appear, however, that fluids are so readily transmitted along this tissue (probably on account of its greater compactness) as they are through the softer parenchyma which envelopes it. It can scarcely be doubted that there is in Mosses a regular transmission of fluid absorbed by the roots, towards the leaves; especially as -fre find many of them furnished with that special exhalant apparatus for the transpiration of fluid, which is fully developed in the more perfect plants (chap, vii.) — In the Ferns, the evolution of a true woody stem proceeds to a much greater extent ; and in this is found a vascular structure, scarcely differing from that of the PhanerogamiaJ Although little has been observed as to the circulation Fig. 109. of sap in this group, it can scarcely be doubted that the fluid absorbed by the roots ascends to the leaves, as in Flowering-plants; and it appears that, as in the least actively- vegetating states of the latter, the sap ascends rather through the cellular parenchyma, than through the ' scalariform ves- sels,' which generally, if not always, contain air. 200. We shall therefore pass-on at once to describe the circulation in the Phanero- gamia, in which it has been more fully investigated ; and we have first to speak of the ascending current, which is produced by the movement of the fluid that is absorbed at one extremity of the axis, towards the leaves by which a large proportion of it is ex- haled at the other. Each Longitudinal section of Stem of Italian Heed : — a, cells of the pith ; b, fibro-vascular bundle, containing, 1. annular duct; 2. spiral duct; 3. dotted duct with woody fibre ; c, cells of the integument. annual layer that composes the wood of the stem of Exo- gens, consists of woody fibre ducts, intermixed with less of cellular and more or tissue (Fig. 109); the vessels being usually situated at the inner part of the ring, and the fibrous tissue, which is not formed until later in the year, lying externally to them. The vessels have usually the greatest diameter in long slender stems, belonging to plants of active vegetation, in which the sap has to be conveyed with rapidity to a con- -^ * CIRCULATION IN VASCULAR PLANTS 215 siderable distance, as in the Yine and the Clematis ; and they are usually larger, also, where the stem is dense, as in the Oak, Elm, Mahogany, &c, than where its softness and laxity allow the whole texture to convey fluid more readily, as in the Pine tribe, which is destitute of any distinct sap-vessels, or in herbaceous plants, in which they are usually small in proportion, or in the young shoots of woody branches in which the inter- cellular passages abound. In the latter it is probable that the cellular parenchyma always affords the principal channel for the ascent of the sap ; and even where the vessels are large, it appears from recent experi- ments to be only when the sap is ascending rapidly, that they take part in its conveyance, their tubes being occupied at other times by air alone, which is then displaced. * The deposition of the products of secretion, which o-ives strength and firmness to the duramen, destroys or greatly diminishes its power of transmitting fluid ; and it is consequently through the external layers, which constitute the alburnum or sap-wood, that the movement of fluid chiefly takes place.— Of the precise course of the ascending sap in Endogens, we have no certain knowledge ; there can be little doubt, however, that it is conveyed through all the tissues of the stem which are not consolidated by interstitial deposit, but more especially by the ducts when these are peculiarly large and open, as is especially the case in long, firm, slender stems, whose leaves are borne at a considerable distance from the roots. Of this arrangement, the various species of Calamus or < reed-palm ' (one of which furnishes the well-known ' rattan- cane') present most characteristic examples; the ducts being of great size and freely pervious through considerable lengths, whilst the remain- ing tissues of their wiry stems are so dense as to be quite unfit for the conveyance of fluid, t 201. The cause of the ascent of the sap in the stem has Jong been a disputed question amongst physiologists ; some attributing it altogether * See Hoffman ' On the Circulation of the Sap in Plants, ' in " Botanische Zeitung, vols. vi. viii., translated by Mr. Henfrey in "Scientific Memoirs," 1853. T It has been affirmed by Prof. Schleiden, that the idea of the ascent of the sap through vessels is altogether a fiction, created by the imagination of those who have desired to find in. Plants the analogues of the Animal functions. But the Author cannot help believing that the desire of that distinguished Botanist to establish the entire absence of analogy between the two kingdoms, has much to do with his somewhat dogmatic denial of a movement ol fluid through vessels in Plants. And whilst the Author is far from denying that the cells woody fibres, and intercellular passages of the stem, all assist in the transmission of fluid from the roots towards the leaves, he cannot but think that the ducts, when fully developed, are the special channels by which this transmission is effected. How else could the ascend- ing sap find its way through the wiry stems of the Calamus rudentum, or cable-palm, which are sometimes 500 feet long ?— The experiments of Honninger ('Botanische Zeitung,' 1843) upon the absorption of ferro-cyanide of potassium, the course of which upwards through the stem was afterwards tested with sulphate of iron, led him to the conclusion (which has been adopted by Link and other distinguished Botanists) that it is only through the tubular tissues of Vascular plants, that fluid ascends. His experiments, however, were chiefly made upon species in which, for the reasons stated in the text, the vessels afford a much readier channel for the sap, than do the other tissues. On the other _ hand, Mr. Eainey concluded from experiments of a very similar kind, made with bichloride of mer- curv that the ascent of sap chiefly takes place in the intercellular passages ("Experimental Inquiry into the Cause of the Ascent and Descent of the Sap," 1847).— The Author cannot but believe that the discrepancies of these and numerous other observations are partly due to differences in the structure of the stems of the respective species upon which they have been made, and partly (as the results of Dr. Hoffman's experiments indicate) to differences in the epoch or activity of vegetation. I I II! ■ 216 OF THE CIRCULATION OF NUTRITIVE FLUID. to mechanical influences, and some regarding it as a purely vital (and therefore completely inexplicable) phenomenon. A very simple .experi- ment will show that two sets of causes must be in constant operation. If the top of a young tree be cut-off in the spring, und the divided extremity be immersed m water, it will absorb a sufficient quantity of fluid for the temporary supply of the leaves; whilst, on the other hand, the portion of the stem left in the ground will continue for a time to discharge the fluid drawn-up by the roots. It is then evident, that the propulsive power of the roots, for which we have already endeavoured to account (§ 177), is a partial, but not the entire, cause of the ascent of the sap in the stem; since the latter will continue by simple imbibition, when the open extremities of the vessels are placed in fluid, provided that the functions of the leaves are sufficiently active to occasion a demand for it. Moreover there would seem no reason why the spongioles should not be as capable of absorbing fluid in the winter as in summer; and if the ascent of the sap depended entirely upon them, we should expect that it would be continued. That they are thus capable has been frequently shown, by grafting a shoot of an evergreen upon a stock whose leaves are deciduous ; it being found that the uninterrupted continuance of the demand meets with a corre- sponding supply. A still more striking experiment is to train a shoot of an out-door vine, or other plant, into a hot-house during the winter ; the unusual warmth will cause an immediate development of the buds, for which a supply of nutriment is required; and this is derived from the roots, whose usual torpidity at this season is thus remarkably interrupted. Careful examination of the first movement of the sap in spring, also leads to the same result ; for it is now ascertained that the upward flow begins near the buds, and that it may be progressively observed in the branches, trunk, and roots, — the latter not commencing their action, until the super- incumbent column has been removed. It can scarcely, then, admit of a doubt, that the demand for fluid, occasioned by the vital processes which take place in the leaves, is the essential cause of the motion of the sap in the higher parts of the tree ; and that the propulsive power of the roots is principally expended m raising it to the sphere of that influence. It is evident that the quantity of fluid absorbed by the roots, will be propor- tioned to the rapidity of its removal by the leaves above; just as the con- tinued rise of oil in the wick, by simple capillary attraction, is regulated by the rate of combustion at its apex. 202. It has been commonly supposed that the ' crude sap' of the ascending current is entirely unfit to nourish the growing tissues ■ and that they derive the materials of their support from a descending current of ' elaborated sap,' which is prepared in the leaves, and is thence returned by a distinct set of vessels to the axis, through which it is transmitted This doctrine, however, can even as far as the extremities of the roots. by no means be admitted in the form here stated ; although it probably contains a certain amount of truth. The ascending current does not contain those elements only which are absorbed from the soil, namely, water, carbonic acid, ammonia, and some mineral ingredients; for, as Prof. Schleiden remarks,* "from whatever part and at whatever time we * cc Principles of Scientific Botany," translated by Dr. Lankester, p. 505. — The whole of the Section entitled ' General phenomena in the Life of the entire Plant ' is worthy of careful study by those who are capable of supplying by their own knowledge what is left unsaid by its learned Author. » "• • DISPERSION OP ELABORATED SAP. 217 examine the sap of a plant, we find that it contains organic principles which cannot come from the soil, because they do not exist there ; such are sugar, gum, albumen, malic, citric, and tartaric acids, &c." The presence of these substances in the ascending current is accounted-for by some Physiologists, on the idea that they were previously contained m the tissues, and have been taken-up by it in its progress through them; whilst by Schleiden and others it is affirmed, that they must have been generated by the assimilating action of these tissues upon the materials drawn-in by the roots.* "Whichever of these two views is the correct one (and it is possible that both may be partly true), it is certain that the ascending current of sap must be capable of affording nutriment to the growino- tissues, in so far as it holds gum, sugar, and albumen in solution; and hence there is no sufficient reason for looking to a supply of < elaborated sap,' afforded by a hypothetical descending current, as their sole pabulum. But, on the other hand, there appears sufficient evidence that the leaves are the chief agents in the introduction of carbon into the system, by the 8 pounds § which the new tissues of the plant are formed, is prepared by their instru- mentality, than by any other means. These organic compounds must be dispersed through the axis ; and there appears strong reason to believe that this dispersion is chiefly effected by the cellular portion of it, and especially, in Exogens, by the bark (with which the leaf-stalks are in very intimate connection) and by the medullary rays. By the bark, these compounds are especially conveyed to the interspace between the liber and the alburnum, in which the formation of new wood takes place ; whilst by the medullary rays they are carried-in towards the centre of the stem, and afford the means of consolidation to the duramen. It is in accordance with this view, that if a ring of bark be removed from a stem, the parts above the ring undergo an unusual amount of increase, whilst those below the ring are comparatively atrophied. There is no reason to suppose, however, that this dispersion is effected by anything like a current ; still less, that any special system of vessels is provided for conveying back the ' elaborated sap' from the leaves to the stem. Its transmission appears to be simply a process of imbibition, taking place between contiguous cells, whereby each communicates to the rest a por- tion of the nutritive materials with which it is charged ; every one making that use of them which is in accordance with its own endowments.— Additional evidence in favour of the view here advocated, will be given hereafter (chap, viii.) * — * * 203. observation circulation of peculiar juices does take place in certain parts of particular tribes of plants, through that system of anastomosing vessels, which has been termed laticiferous (Fig. 110). This cyclosis has been only observed in plants with c milky' juices, that is, in those which have a i latex 5 ren- dered opaque by the presence of floating particles of resin, caoutchouc, or other substances ; and it is altogether questionable, whether this i latex' * It is difficult to understand, however, how this process can be effected by the cells of the interior of the stem and roots, secluded as they are from the direct influence of light; without which (we have every reason to believe) no direct production of organic compounds from inorganic elements can take place. : 218 OF THE CIRCULATION OF NUTRITIVE FLUID. is not peculiar to the plants with milky juices, and is not rather to be considered in the light of a special secretion, than as a nutritious fluid. I When the laticiferous Fig. 110. B Laticiferons vessels : — a, their formation from cells ; b , network of milk-vessels from the stipule *of Ficus elastica. vessels are cut or bro- ken across, a flow of fluid takes place from the wounded part; and if a piece of the bark or leaf of a ' milky' plant be cut out and placed under the mi- croscope, the fluid con- tained in the vessels will be seen in rapid movement throughout. This, however, has no relation to the true circulation, which was first described by Prof. Schultz as taking place in the laticiferous ves- sels of an uninjured part.* The movement seems to take place in contiguous all directions, the currents often running contrariwise vessels. Sometimes one of these currents may be observed to stop, its cessation being preceded by a temporary oscillation ; it afterwards recom- mences, or a new current is established in a contrary direction. The rate Nova Acta Acad. Nat. Curios.," vol. xviii., and "Ann. des Sci. Nat.," 2 e Ser Botanique, torn, vn— The statements of Schultz have been called in question by many distinguished Botanists, amongst others by Prof. Schleiden, who have regarded the move- ments described by him as the result of injury to the vessels, permitting the discharge of their contents, and consequently establishing a current towards the point of exit. So many competent observers, however, have satisfied themselves that such is not a sufficient ex- planation, that the existence of a regular movement of the latex in certain plants must in the Author's opinion, be regarded as an established fact, whatever be its decree of o-ene- rality. The latest recorded observations on this point are those of Prof. Balfour • which are to the following effect. — "From observations made last summer, I am disposed to agree with Schultz' s statements. It is true, as Mohl remarks, that any injury done to the part examined, causes peculiar oscillatory movements, which speedily cease. Thus if the young expanded sepal of the Celandine is removed from the plant and put under the microscope, or if the inner lining of the young stipule of Ficus elastica be treated in a similar manner, very obvious motion is seen in the granular contents of the vessels, and this motion is affected by pricking the vessels or by pressure. In order to avoid fallacy, however, I applied the microscope to the stipules of Ficus elastica while still attached to the plant, and uninjured ; and I remarked that, while pressure with any blunt object on the stipule caused a marked oscillation in the vessels, showing their continuity, there could, nevertheless, be observed a regular movement from the apex towards the base', inde- pendent of external influences, when the stipule was simply allowed to lie on the field of the microscope, without any pressure or injury whatever. This movement continued for at least twenty minutes during one of the experiments, and I have no doubt might have been observed longer. It is of importance to distinguish between those molecular movements which are caused by injury and pressure, and those which depend on processes going on in the interior of the living plant. My experiments are by no means complete ; but they lead at present to the adoption of Schultz's opinion relative to the existence of the cyclosis " " Manual of Botany," 1849. j FORCES PRODUCING MOVEMENT OP NUTRITIVE FLUID. 219 of movement is greatest in parts which are in progress of development, other things remaining the same j it is also accelerated, -within certain limits, b Y warmth ; and is retarded or entirely brought to a stop by cold, recommencing on the renewed application of warmth. A strong electric shock puts an end to it immediately. 204 The resemblance between this movement in Plants and the capil- lary circulation in Animals, makes it a point of peculiar interest and importance to determine the nature and source of the forces by which the former is sustained. It is quite obvious that the movement cannot be due to any vis d, tergo ; both because it is far from being constant m its direction in particular vessels, and because there is no organ to supply a propelling force, which could extend itself through such a complex system of anastomosing canals. Nor, again, can we attribute it to any vwafronte, like that which takes part in producing the ascending current from the roots towards the leaves. It is certain, too, that no such contraction of the laticiferous vessels themselves takes place, as could be effectual m propellin- the fluid through them. Further, the movement continues for some time in parts that have been completely detached from the rest, and on which neither vis dt tergo nor vis afronte can have any influence. On the other hand, the facts stated in the preceding paragraph all indicate that like the 'rotation' within the individual cells of Plants (chap, viii.), the movement of fluid within the laticiferous vessels (whatever may be its purpose in the vegetable economy) is intimately connected with the formative operations of the part, and is dependent upon forces which arise out of these. The manner in which they become so, is the next object of our enquiry; and on this subject, some views have been put forth by Prof. Draper,* which seem to help towards an explanation ol the phenomena. . . . , -,. 205. It is capable of being shown, by experiments on inorganic bodies, that, if two liquids communicate with each other through a capillary tube, for the walls of which they both have an affinity, this affinity being- stronger in the one liquid than in the other, a movement will ensue ; the liquid which has the greatest affinity being absorbed most energetically into the tube, and driving the other before it. The same result occurs when the fluid is drawn, not into a single tube, but into a network ol tubes permeating a solid structure ; for if this porous structure be pre- viously saturated with the fluid for which it has the less degree of attiac tion, this will be driven out and replaced by that for which it has the greater affinity, when it is permitted to absorb this. Now if, m its pas- sage through the porous solid, the liquid undergo such a change that its affinity be diminished, it is obvious that, according to the principle just explained, it must be driven out by a fresh supply of the original liquid, ana that thus a continual movement in the same direction would be pro- duced. Now this is precisely what seems to take place in an organised tissue' that is permeated by a fluid, between whose particles, and those of the tissue which it penetrates, affinities exist, which are concerned in the formative changes that take place during its circulation. Por these affinities are continually being newly developed by acts of growth, as fast * " On the Forces which produce the Organisation of Plants," by John "William Draper, M.D., New York, 1844 ; pp. 29 et seq. \ I ; »■ » IE I M I i 220 OF THE CIRCULATION OF NUTRITIVE FLUID. as those which previously existed are satisfied or neutralised by the changes that have already occurred ; and thus in the circulation of the nutritive fluid, there is a constant attraction of its particles towards the walls of the vessels, and a continual series of changes produced in the fluid as the result of that attraction. The fluid, which has given up to a certain tissue some of its materials, no longer has the same attraction for that tissue ; and it is consequently driven from it by the superior attraction then possessed by the tissue for another portion of the fluid, which is ready to undergo the same changes, to be in its turn rej ected for a fresh supply. Thus in a growing part, there must be a constantly-renewed attraction for that portion of the nutritive fluid which has not yet tra- versed it ; whilst, on the other hand, there is a diminished attraction for that which has yielded-up the nutritive materials required by the particular tissues of the part ; and thus the former is continually driving the latter before it. But the fluid which is thus repelled from one part, may still be attracted towards another, because that portion of its contents, which the latter requires, may not yet have been removed from it; and in this manner the current may be maintained through the whole capillary net- work, until the liquid has been entirely taken up by the tissues which it permeates. The source of the movement being thus attributable to the formative actions to which it is subservient, it is obvious that it must be affected by any external agencies which quicken or retard these ; and it is thus that the influence of heat, cold, and electricity upon the rate of the flow seem most readily explicable. — These principles will be hereafter shown to have a most definite application to the phenomena of the ' capil- lary circulation' in Animals (§ 251). 206. The development of the Circulating system during the growth of Vascular Plants, has not yet been made an obj ect of special attention ; the general facts with which we are acquainted, however, correspond exactly with the principles which have been previously stated.— As the absorption of nutriment by the embryo within the ovule appears to take place through the whole surface, there is no transmission of fluid from one portion to another ; nor do we find, even at the period of the ripening of the seed, any distinct vascular structure. As far as its circulating system is con- cerned, therefore, the young plant, at the commencement of germination, is on a level with the simpler cellular tribes. During the rapid longitu- dinal development, however, which then takes place in the stem and root, there is of course a peculiar transmission of fluids in those directions • and this appears to be at first performed, as in the stem of the Fungi, by elongated cells and intercellular passages. It is not until the true leaves are expanded, that we find true ducts in the stem, formed by the coales- cence of linear series of cells,* mostly containing spiral fibres or some other secondary growth in their interior ; and it is very interesting to remark, that these ducts in young plants often present the appearance which is characteristic of the Ferns, having the spiral fibre more or less regularly disposed within them (Fig. 109, i, 2); whilst, after the stem has ceased to increase rapidly in length, these canals are converted into dotted ducts (Figs. 109, 3). The anastomosing vessels of the latex in like manner * This view of the mode in which the ducts of Plants are formed, based on a comparison of the various forms which they present, has been confirmed by the recent observations of Dr. Hobson on the history of their development. (" Ann. of Nat. Hist.," vol. xi. p. 72.) GENERAL CONDITIONS OF CIRCULATION IN ANIMALS. igulai 221 several points, and not, as in the formation of dncts, by their extremities alone. This change is represented in progress m Fig. 1 1 0, A. 3. Circulation in Animals. 207. In following the evolution of the Circulating system through the Animal scale, it will be easy to discover its conformity to the same gene- ral plan, as that which has just been traced-out in the Vegetable kingdom. In proportion as the power of absorbing aliment is restricted to one part of the surface, whether external or internal, does it become necessary that means should' be provided for conveying the nutritive fluid to distant organs ; not merely that it may furnish the supplies which they are con- stantly requiring for the maintenance of their respective structures, and for the manifestation of their vital properties ; but also that it may itself undergo certain changes, which are essential to the continuance of its characteristic qualities. Not only does the Circulation of fluid through the system enable the new materials to be deposited m their appropriate situations but it also takes-up and removes the particles, which, having manifested a tendency to decomposition, are no longer fit for the offices which they previously contributed to perform ; so that, by the various processes of elimination, these may be separated from the general mass, and be either appropriated to some other purpose in the economy, or be altogether carried out of the structure. The excretion of carbonic acid by the Respiratory apparatus is one of the most considerable and im- ' portant of these processes ; and it will be found that the distribution of the Circulating system has always an express relation to the conditions under which this is performed. In fact, so peculiar is this adaptation m the higher Animals, that many have considered the sanguiferous system under two heads,— that belonging to the general circulation of nutritious fluid through the body,— and that which performs the respiratory circu- lation, conveying the blood, which has been rendered impure _ by the changes it has previously undergone, to the organs where its physical and vital properties are to be renewed by contact with the air. Respiration differs not in kind, however, from the other functions of purification, but only in its relative importance ; and although in warm-blooded V erte- brata, whose nervo-muscular energy can only be maintained m full vigour by a constant supply of oxygenated blood, its cessation even for a short time is fatal, there are many amongst the lower classes, in which # it can be suspended for a considerable period with impunity, and m which the increased amount of other secretions appears to counterbalance the dimi- nution in its products. We find too, even in the highest Vertebrata, peculiar modifications of the circulating apparatus in connection with other secreting organs, as the Liver in Mammalia, and the Kidneys in Birds • and yet more remarkable modifications of the same nature are elsewhere found : so that it should rather be stated as a general fact, that, in proportion to the variety of the organs, and -the importance of the functions they perform, is the special adaptation of the Circulating appa- ratus which supplies them,— than that it undergoes modification according to the conditions of the respiratory system alone, as Cuvier maintained. In proporti on as the function of Absorption is restricted to one part of I II l! ! ' m '* 222 I t >1 OF THE CIRCULATION OF NUTRITIVE FLUID. the surface, that of Respiration will be limited to another ; and the pro- cesses of Nutrition, and the formation of Secretions, will go-on in parts of the structure distant from both ; and all these must be brought into harmony by the Circulating system, the arrangement of which will evi- dently vary from the most simple to the most complicated form, according to the number and variety of the actions to which it is subservient, and the vigour with which these are performed. Muscular the greatest, the arrangement of the Circulating apparatus, and the rate of the movement of blood through it, appear to have special reference to the demand for oxygen created in the discharge of the Animal functions, and to the necessity for the removal of the carbonic acid generated in the same processes ; whilst there is evidence that the organic operations of growth and development might be carried-on by means of a much less rapid flow, and by a less perfectly-oxygenated blood. A remarkable example of this principle is presented by the condition of the Circulation in the class of Insects, in which the activity of the animal functions is relatively greater than in any other group ; for we find that the movement of their nutritive fluid, which is subservient in them to nutrition alone, is comparatively slow and feeble (§ 224), the very active aeration of their nervo-muscular apparatus being accomplished, not so much by the medium of their blood, as by the penetration of air-tubes into the tissues themselves (§ 302). _ 209. The Circulating apparatus of Animals, at least where it is dis- tinctly developed, differs in one important particular from that of Plants We have seen that, in the latter, the sap which has been elaborated in the leaves is dispersed through the fabric, giving-up its nutritive con- stituents to the parts to which it finds its way (§ 202) ; and if any of it mixes with the ascending current, and circulates a second time through the system, the amount of this is comparatively small. In the higher Animals on the contrary, we observe that the same fluid is repeatedly transmitted through the body ■ the alterations which are effected in one part of its course by the withdrawal of materials for particular processes of Nutrition, being counterbalanced in others by nutritive operations of some different kind (see chap, viii.), as likewise by those of Respiration and Secretion, and by the continual admixture of new alimentary mate- rials. 210. In all the higher forms of the Circulating apparatus, again, we find a central organ of impulsion, the Heart, into which the fluid returned from the various parts of the body is poured, and on whose contractile force the maintenance of the current chiefly depends. From this it passes-out by one or more large trunks, which convey it to the several organsand tissues; these are called Arteries. The arteries gradually subdivide into ramifying vessels, which, repeatedly undergoing the same change (Fig. Ill), terminate in a complex system of anastomosing (inter- communicating) tubes, of nearly uniform size, which are termed Capil- laries. It is m these only, that the blood comes into sufficiently intimate relation with the tissues which it supports, or by which secretions are elaborated from it, for the performance of chemical or vital reactions be- tween them ; so that we may consider the function of the arteries to be the simple conveyance of the nutritive fluid from the central organ of J -T . ARTERIES, VEINS, AND CAPILLARIES. 223 impulsion to this network of capillaries, which exists in most of the living tissues of the body, and in near proximity to the remainder. Jiven the Fig. 111. \ a Web of Froq's foot, stretching between two toes, magnified 3 diameters; showing the blood-vessels, and their anastomoses : a, a, veins ; b, b, b, arteries. walls of these trunks are furnished with a distinct set of branches (the mm vasorum), proceeding from neighbouring vessels, for their own nutri- tion. After traversing the capillaries, the blood is received into another series of vessels, formed by their reunion, which are termed Veins; and trunks reservoir —The walls of the Arteries and Veins are formed of several The layers of tissue, which, however, constitute three principal ' coats inner coat is a pellucid structureless membrane, continuous both with that which lines the heart, and with that which forms the sole boundary of the capillaries ; and this is covered on its free or internal surface by a - r .-..'. " The outer coat is simply protective, and is The middle or ' fibrous ' coat, how- layer of epithelial cells. formed of condensed areolar tissue. ever, which is much thicker in the arteries than m the veins, is partly composed of yellow elastic tissue ; and partly of non-striated muscular fibre By the agency of the former, which is especially abundant m the larger arteries that receive the blood direct from the heart, the inter- mitting i ets in which the fluid is at first propelled by its contractions, are gradually converted into a continuous stream. The latter, on the other hand is more abundant in the smaller arteries, and appears to tary heart but its main purpose seems to be, to regulate the diameter of the vessels in doin<* which it is probably influenced in some degree by the Sympathetic system of nerves that is minutely distributed upon it. Of rapid alterations in their diameter, which, being emotional m their source, I 1 1 i i li J 224 OF THE CIRCULATION OF NUTRITIVE FLUID. can be only referred to the instrumentality of the nervous system, we have a characteristic example in the act of 'blushing.' — The capillary vessels, too, in the higher animals, possess a distinct wall, which is con- tinuous with the lining membrane of the larger vessels ; and they are thus to be considered in a different light from that of a mere system of passages excavated in the substance of the tissues. 211. The trunks and branches of the Blood-vessels appear to be formed, in the first instance, like the ducts Fig. 112. Formation of Capillaries in tail of Tadpole :— a, a, capillaries permeable to blood ; b, b, fat gra- nules attached to the walls of the vessels, and con- cealing the nuclei ; c, hollow prolongation of a ca- pillary ending in a point ; d, a branching cell, with nucleus and fat granules, communicating by three branches with capillaries already formed ; e blood- corpuscles, still containing granules of fat/ cence of cells arranged in linear series ; those of moderate size taking theii origin in single or double files of such cells, whose coalesced walls form the primitive simple membranous tubes of these vessels ; whilst the principal trunks, out § of aggregations of cells, of which those in the interior liquefy to form the cavity, whilst those on the periphery are metamorphosed into the fibrous and other tissues of which their more substantial walls are composed. The first Capillary network, however, seems to be formed by the coalescence of pro- longations of stellate cells (Fig: 112), which come into contact either with the walls of vessels already existing, or with each other; and when their cavities have become continuous with those of vessels previously tra- versed by blood, they too begin to receive that fluid, then enlarge, and at last form a regular network of tubes,— their cellular origin however, being still inclicatecf by the presence of nuclei in their walls.* The subsequent deve- lopment of capillaries, on the other hand, usually takes place by out- growth from the vessels previously formed; in the mode thus described by Mr. Paget, t « Suppose a line or arch of capillary vessel, passing below the edge or surface of a part to which new material has beeS superadded. The vessel will at first present a slight dilatation in one, Bati e ' e ih ? . oh ?f™ tio f °lJ^*m&er oth as to the arrangement of this system, and as to it: degree of development, a considerable difference seems to exist among the principal sections of the class. 216. In the Asterias, in which the digestive cavity is prolonged into the c rays' or lobes of the body (Fig. 37), a ' mesenteric' trunk is found lying on the surface of each of the radial cseca ; and the several trunks, con- verging from the rays to the central disk, unite with other branches from the stomach, to form a circle or vascular ring around the upper part of the disk. trance to the stomach on the lower surface, by means of a vertical descending vessel, which Tiedemann found to possess muscular irritability, and regarded as the rudiment of a heart ; whilst, from the lower ring, other vessels proceed which are distributed through the disk and rays. No capillary network, however, nor even any system of minute ramifications, has yet been traced in continuity with these trunks. — In the Echinus, two vascular trunks are found to run along the intestine, one of which is supposed to be venous, the other arterial. The supposed mesenteric vein passes towards the oral orifice, where it terminates in a vascular ring, with which is connected a contractile vesicle, resembling that of Holo- thuria (Fig. 40, p), but of more prolonged form. are given-off five vessels which supply the parts connected with the mouth, five trunks which pass along the ambulacral regions in the mem- brane lining the general cavity of the body, and another trunk, appa- rently arterial, which runs along the border of the intestinal tube. The ambulacral trunks converge again towards a vascular ring that surrounds the anus ; whence also are given off trunks for the supply of the ova- In the From this oral ring ries. Holothuria, the vascular system attains a far higher degree of development ; for the trunks subdivide into ramifications of greater minuteness, and the fluid they contain is therefore more exten- sively distributed. ^ The same general plan may be traced, as in the pre- ceding cases ; but with variations which seem to have reference to the more special development of the respiratory apparatus in this order, as well as to its transitional character. The intestinal system of vessels consists of a long trunk, doubled on itself, which passes along the external margin of the intestine (Fig. 40, v e), and which ramifies minutely upon its surface ; its two inflexions being directly connected by the anastomotic branch va 9 v af. The ramifications of this trunk, along the upper part of the intestine, seem to terminate directly in those of the internal intes- tinal vessel vi; but those of the middle part of the alimentary canal are continuous with those of the mesenteric trunk vm; whilst between this last trunk and the internal intestinal trunk, is a series of minute plexuses v r. The course of the blood through this system of vessels has not been positively determined ; but it would seem probable that one of the intes- tinal trunks is venous and the other arterial, and that the minute distri- bution of the blood-vessels upon the walls of the intestinal canal has reference to the introduction of fresh nutritive materials into the sanguiferous circulation; whilst it can scarcely be doubted that the mesenteric plexus is subservient to respiration, though it does not come *» \i CIRCULATION IN ARTICULATA. 229 this system of vessels, there is § Besides another, consisting of an annulai trunk (va), which surrounds the mouth (as in Echinus), and sends off branches to the buccal apparatus and to the tentacula (t), as also to a set of tegumentary vessels (vl, vl'), which supply the general surface. In con- nection with the oesophageal collar is found a saccular dilatation (p), which seems to represent the pulsatile vessel of the Echinus ; hut whether it possesses a similar contractile power, is not known. The connection of this set of vessels with the preceding has not yet been made out. 217. The Articulated classes are usually regarded as inferior to the Mollusca in the evolution of their circulating apparatus ; and it certainly never presents the same concentrated condition in the former group, as in the latter. There are some extensive groups in this sub-kingdom, as already remarked (§212), in which there does not appear to be any other circulation than such as may take place in the ' general cayity of the body •' the fluid in its lacunae being continually put m motion by the movements of the animal, and being thus driven from one part of the body into another. Amongst the lowest Articulated animals which have been until recently supposed to possess a proper blood-vascular system, this can no longer be regarded in its former light ; and we shall see that even in the case of the Annelida, a considerable modification of previous views may not improbably be required. Where a Sanguiferous system unquestionably exists, it appears to be constantly formed upon the following plan. A vascular trunk passes along the dorsal region, in which the blood passes from behind forwards, being propelled by the contractions of its walls ; this trunk, in its typical development, is divided by transverse valvu- lar partitions into as many chambers as there are segments of the body ; and by the successive contractions of the walls of these chambers, the circulating fluid, which enters at the posterior extremity of this 'dorsal vessel,' or which is received into it at any portion of its length, is pro- pelled forwards through a principal trunk which is continuous with its anterior extremity, as well as through smaller vessels proceeding from the chambers to their own segments. This arrangement, however, is For in the lowest forms of this dorsal seldom completely carried-out. vessel/ although it is distinctly contractile through its whole length, there is no division into chambers ; whilst in the highest, a considerable portion of it is destitute of propulsive power, only a few chambers or even but a single one, being endowed with muscular contractility, lhe blood propelled forwards and distributed by the dorsal vessel, is collected and returned to the posterior part of the body by a ventral trunk.— As in most other parts of the nutritive apparatus of this group, it may be observed that a very exact bi-lateral symmetry prevails, alike in the central organs, and in the peripheral distribution of the blood-vessels. 2 1 8. A lar o*e share in the distribution of nutritive materials through the system of the Annelida, is undoubtedly taken by the ' general cavity of the body •' which not only surrounds the alimentary canal throughout its whole length, but also sends prolongations into the greater number of the appendages that are given off, as well from the head, as from the trunk. The fluid contained in this cavity is richly corpusculated and coaoailable • and it is kept in continual movement by the contractions aiuf extensions of the different segments of the body, which most of these ■ v ; II ' 230 OF THE CIRCULATION OF NUTRITIVE FLUID. f 1 animals are incessantly performing. The visceral portion of the cavity- is sometimes almost completely divided by transverse inflections of its lining membrane, into segmental chambers ; but these always communi- cate with each other more or less freely, so that there is a continuous passage for their contained fluid from one part of the body to another. This fluid, when seen in motion within the cirrhi and other appendages into which it freely passes, has been usually considered as blood; but it is obviously to be regarded in the same light as the ' chylaqueous fluid' of inferior animals ; yet when its composition, the universality of its dif- fusion, and the obvious provisions for its aeration, are together taken into account, it seems impossible to resist the inference, that it is scarcely less essentially subservient to the nutrition of the fabric, than is the blood of higher animals. In certain degraded Annelida, indeed, no vascular system exists ; and it is obvious that the movement of the chylaqueous fluid through the visceral cavity and its prolongations must there be the sole representative of the circulation.* 219. In by far the greater number of animals belonging to this class, however, there is a vascular system of very remarkable character; con- sisting of a set of trunks and vessels, usually furnished with a multiplicity of organs of impulsion, and subdividing into ramifications of great minute- ness. The extent and mode of distribution of this vascular system differ greatly in the several sections of the class; but they obviously have reference in great degree to the amount of specialization, and to the mode of arrangement, of the Respiratory apparatus. The fluid which they contain is usually coloured, being very commonly red, sometimes green, more rarely of a yellowish or brownish hue, and occasionally colourless ; but it seldom or never contains corpuscles; and in this respect, therefore, it departs so widely from the usual character of a nutritive fluid, as to sug- gest a strong doubt whether it be truly blood, and whether the vessels which contain it ought to be regarded as the representatives of the sangui- ferous system of higher animals. This doubt derives additional force from the greater degree of resemblance which this vascular system of Annelida bears to the ' aquiferous system' of the Trematoid Entozoa (§ 286), the Nemertine Worms, &c. (the connection between the two being established by the Leech and its allies), than to the typical form of the sanguiferous system of the Articulated sub-kingdom as it exists in Myriapoda ; and from the very anomalous character which it would present, if really sangui- ferous, in being the only known example in the whole Invertebrated series, of' a blood- vascular system not communicating freely with the general cavity of the body. It seems far from improbable, however, that the func- tion of this apparatus is essentially respiratory ; and that it may perform the function of the sanguiferous system in distributing a highly aerated fluid through the body for the oxygenation of its tissues, whilst the move- ment of the chylaqueous fluid through the general cavity answers the purposes of nutrition, t * See the Memoirs of M. de Quatrefages and of Dr. Williams already cited ; and the < Etudes sur les Types inferieurs de l'Einbranchement des Anneles,' by the former of these observers, in the " Ann. des Sci. Nat.," 3 e Ser., Zool., Tom xiv. t The question above stated with regard to the real nature of the vascular system in the Annelida, was first suggested to the Author by his friend Mr. T. H. Huxley; whose doubts on this point appear to him fully justified by the evidence which will be adduced hereafter, in regard to the \ water -vascular system' of the Entozoa and their allies. There CIRCULATION IN ANNELIDA 231 220 In the Leech and its allies, which constitute the division of this class that is most nearly allied to the inferior Annulosa, four prmcipal longitudinal trunks may be ^^^i^^J^\S^ and two lateral. trunks pulsive power \ and they appear to contract and dilate 'alternately with each other They send off branches which form a plexus underneath the skin • and this distribution of the circulating fluid seems to have reference chiefly to its aeration, there being no more special provision for that nurnose The lateral trunks communicate freely with each other by transverse arches, into which a continual stream of blood ^ V^redbj the contraction of one or other of them ; and this is distributed, by vessels proceeding from the arches, to the intestinal canal, the generative organs, and also to a set of looped vessels which proceed to a row of saccub l on each side of the body, that have been variously regarde f as resp^tory organs, as muciparous follicles, and „ «*£« «£ ,7^™£ the t Jo median (dorsal and ventral) trunks; and m this plexus it seems to exlcutean oscillatory movement from one side to the other, by the aHemating impulses of the lateral trunks, into one or other of which it will at last find its way, to recommence its movement through the body generally^ ^ Annelida, the most constant parts of this apparatus are the dorsal and ventral trunks; the lateral vessels, however, often exist, although they are seldom of the same relative importance as in the Suctoria ; and the dorsal and ventral trunks are themselves some- times double along a part or the whole of their ■ course, running on _the two sides of the median line, or at a little distance from it, like the longitu- dinal vessels in the Entozoa (Fig. 136). Moreover, we generally find special contractile dilatations on some part of the vascular system ; sometimes only a single one exists ; but more commonly they are greatly multiplied. Our acquaintance with the circulation in this group, has chiefly resulted from the skilful observations of MM. Milne-Edwards and De Quatre- fages.t— In the Terebella, whose gills are arborescent, and situated round the head (Fiji. 113,*, k), there lies in the anterior part of the body, on the dorsal aspect? a large'trunk (I), which receives at its posterior extremity, are, it is true, some very cogent objections against ^^^^T^s^ Annelida in the same category; and more ^specially, the *W**rt f re sentedby all the pxternal onenin* in the neighbourhood of the anus or elsewhere, as is presented uy an tne oraluary f™ of the wate^-vascular system. But it is by no means certain that such comScatLs do not sometimes exist in the Annelida ; and there are undoubted examples among\he Entozoa, of vessels which must be ranked as belonging to the water-vascular Astern though destitute of any external orifice. -The Author has thought the expression of Ee doubts to be called-for by the present state of the enquiry, which further research will Tobably soon elucidate ; but he has not felt justified in venturing upon so bold a step Is the removal of the supposed sanguiferous system of Amwhda out of the category of the CirculatiS ^Apparatus, although he has felt no hesitation as to this step m the case of the ^ti^Z^^ differs from that of M. Duges, who first described the circula- tion I Z f mrudunito, is given on the authority of M. Gratiolet "Ann. des Sci. Nat.,' ^^l^Z^£h^ former in the "Ann des Sci Nat'" 2* Ser., Zool., Tom x : and the resume of the researches of the latter already referred to. '* it i ill IP I 232 OF THE CIRCULATION OF NUTRITIVE FLUID. from a venous sinus (n) surrounding the oesophagus, the contents of an extensive vascular plexus, that has ramified on the walls of the intestine. Fig. 113. Fig. 114, hJ < ■ * J Circulating Apparatus of Terebella con- chilega: — a, labial ring; b, b, tentaeula; c, first segment of the trunk ; d, skin of the back ; e, pharynx ; /, intestine ; g, lon- gitudinal muscles of the inferior surface of the body; h, glandular organ (liver?); i f organs of generation ; j, feet ; k, h, bran- chiae ; I, dorsal vessel acting as a respira- tory heart ; m, dorso-intestinal vessel ; /?, venous sinus surrounding oesophagus; u 1 , inferior intestinal vessel ; o, o f ventral trunk ; p, lateral vascular branches. * .. i\ ■ *& Circulating apparatus of Eunice sanguinea: — a, b, c, an- tenna? ; e, first segment of the i y; /' feet ' 9> pharynx; g\ mandibular muscles ; i, intes- tine; l\ dorsal vessel; Z, superior intestinal vessels ; s 9 their lateral branches; q, ventral vessel; t, its lateral branches ; V, contractile bulbs on these branches; u, branchiae ; x, subcutaneous ve; sels of the back. and on the muscles, integuments, &c. This trunk, or dorsal vessel, propels forwards the blood, which it receives from behind, by irregular contractions. At its anterior extremity it subdivides into numerous branches, of which the principal enter the respiratory organs (k, k), whilst others pass to the head and tentaeula (b, b); so that a large proportion of the blood is aerated, before it is again circulated through the system. The vessels that return it from the gills reunite into a trunk (o, o), which passes down the ventral surface of the body, giving off a pair of trans- CIRCULATION IN ANNELIDA 233 verse vessels to each segment, and then returning along the inferior side of the intestine (V) ; this trunk is of course to be regarded in the light of an artery. The blood is conveyed back into the venous sinus, both from the intestine and from the parietes of the body, by the dorso- intestinal vessels (m), which must be considered as veins, lne pro- pulsion of blood into the gills seems principally due to the contractions of the dorsal vessel, which may here be regarded as a sort of respiratory heart ■ but its motion through the arterial trunk would appear to be partly owing to contractions of the gills themselves, which are occa- sionally seen to take place. The irregularity of these, however requires some supplemental force, such as that which has been already described in the inferior tribes, for the maintenance of a steady current ; and there are many allied species, in which the blood circulates no less energetically, without any such evident propelling agents. 222. In the Eunice (Fig. 114) we find the same general distribution of vessels, but there is an important change in the position m the respira- tory organs, which involves a complete alteration m the character of the different parts of the system. The branchial organs («) are ^concen- trated round the head, but are disposed in < combs along the whole body. The dorsal trunk (?) receives from the dorso-mtestmal vessels {I), as m the former case, the blood which has ramified on the intestines j but this fluid, as will presently appear, is as much arterial as venous. The con- tractions of this trunk are not so regular and powerful as in the lere- bella, and seem to be but little concerned in maintaining the circulation. The vessels into which it divides anteriorly, are distributed only to the head and neighbouring parts ; and, by the reunion of the vessels which return the blood so distributed, the ventral trunk (q) is formed, which here possesses a venous character. The general distribution of its ramifi- cations is very similar to that described in the Terebella, except that it transmits blood to the branchiae as well as to the general system ; but each transverse branch (*) presents a dilatation or bulb (/) near its origin, which seems to propel the blood that enters it, by regular con- tractions, partly through the pectinated branchiae, and partly upon the intestines, cutaneous surface, muscles, &c, after permeating which it re- enters the dorsal vessel. This multiplication of blood-propelling organs is very interesting, when viewed in reference to the general tendency to repetition of parts manifested in this class. It is found that, in tfie Annelida which possess it, the vitality of portions of the body is preserved i »• „i?x_ j/u^ ^wj^ricinrvn nf thn animal, in nierner during a very long time after the subdivision of the animal. tribes? however, this multiplication is restricted to a particular division of the body, as will be presently seen in the Earth-worm. 223. In the Arenicola (Fig. 1 15) we observe another interesting variety in the arrangement of the vascular system, which partly resembles the forms already noticed, and partly conducts us to others which would at first sight appear entirely different. The dorsal vessel (o, o) traverses almost the entire length of the body posteriorly, and it receives, as before, the blood which has circulated on the intestine and external surface, as well as some directly transmitted by the branchiae. It terminates, however, at about the anterior fourth of the body, in a kind of contractile ventricle (n), which answers the purpose of a heart j but it first sends forward branches (x) to the head, the vessels returning from which enter the ventral trunk (*) i i u I,' 234 OF CIRCULATION OF NUTRITIVE FLUID. ; that passes backwards from the propelling cavity. The branches of this I trunk are almost entirely Fis. 115, a -\'Y •it >(. 'if It- 1 > -SV 71 .V K m \~4 .-. - ^- -" 1 3 " ££* a<~^ f? kx ^ ■*-ll X ^ -*%> L U. / wm« a -*» x •5 rrf\ l& $> ] /\r .V. ^^p. wz t7 fcs* s^a %. »*fl ^ 3^S«r ^ ^i*ii IS; ffl iS^ ^^ 7 '"- t-l a - ^ -Y ST $ t^"V- -w si ■ <* _": i ^. .m r. ■ * i^K ■B WtTS, wm is -fl^ > *: Uf 1^ /; * -.-^i t *« ^ $ x Wi'M fflu « wr/AES'' 38 *. ; S~ o 13 V ^f ll M ^ V*" -,t . t" I ■ ■I ft ii... iK.f'-'hti'W. u % < ■ f f awe 'i') B a i - / ' /fc /; w '■ "«*. r X M ri •M L: 'M y# rir // s iiiKi 'In uMA i.M (i :K:'. !■ ■ ^* ^S fS b i inn m !H *^ H -i 7JJ' IM :-.- 5S J If * WW -^i- a l 1 ! I 1 'H r . IL -> IF'* V3 0- C** 1 distributed to the gills — i 13 ); and the blood which is returned from them, is partly transmit- ted to the intestine by the lateral intestinal vessels (p), partly to the integu- ments, and partly to the dorsal vessel direct. The branchiae here, as in the Terebella, seem to exert a direct propelling power on the blood which passes through them. — In the Lmnbricus or ' Earth- worm/ again, we find the dorsal vessel communi- cating with the ventral trunk, not by one con- tractile cavity at its anterior extremity, but by several loop-like di- lated canals, which seem to exercise a similar propelling agency. The waves of blood can be distinctly seen, if the ani- mal be kept without food for a time, until it has discharged the black earth which usually fills its intestinal canal. The blood which is thus for- cibly propelled into the ventral trunk, veyed backwards the body, and distributed to its different organs, especially to the aqui- ferous tubes which take is con- along the place of the respira- of higher Circulating Apparatus of Arenicola piscatorum, as seen from above at a, and as seen from the side at B ; — a, proboscis ; /?, pharynx; c, retractor muscles ; d, dilated oesophagus, or crop; e, csecal appendages ; f t stomach ; g } intestine ; h, muscular partitions surrounding the abdominal portion of the digestive tube ; i 1 to i 13 , thirteen pairs of branchiae ; j, organs of genera- -fcorv trachese tion ; k, setiferous tubercles, and their muscles ; l y secreting J caeca of the yellow matter exuded from the skin ; m, secreting caeca (biliary ?) surrounding the intestine ; n, n, heart ; o, dorsal QTl A -fWym -j-Vipqa i+ iq t»^ vessel; o', abdominal portion of the vessel; jp, lateral intestinal an<1 tTOm tnese 1Z 1S re " vessels ; q, subcutaneous vascular network ; r, branchial arteries turned to the dorsal VeS- and veins ; s, branchial veins returning to the dorsal vessel ; t, ventral trunk ; u 9 u', cutaneous vessels ; x, lateral pharyngeal vessels ; z, labial vascular ring. air-breathing Articulate,; having sel, its aeration Ibeen effected through the medium of the general surface. — The great extent and importance of the capillary system in the l \ I CIRCULATION IN MYKIAPODA, 235 Annelida, compared with the feebleness of the central propelling powers, is an interesting feature in the character of their vascular apparatus, and shows that we have not yet arrived at a condition of the circulating system very far removed from that which it presents m Plants. 224 In the higher Articulata, the Circulating apparatus, instead ol beino- distinctly differentiated from the ' general cavity of the body,' as it seems to be in the Annelida, is always in intimate connection with Insects it Through the whole series of Myriapods, , Arachnida the < blood' and the < fluid of the general cavity' are iden- tical • for the former, in some part of its circulation, escapes into the lacunae between the tissues, and is diffused through the interior of the peritoneal and pericardiac sacs. And m the embryonic condition ol these animals, as in certain degraded forms, the development of whose circulating system x» ^xx^^x.v V ^ A - ^ , there is no other circulation than the movement of chylaqueous fluid to and fro within the visceral cavity. Myri ana iro witnm xuv v^ckw^t^. * - voda as in other parts of their structure, we meet with the condition poaa, as in owiex F . • _ r ~* a « a ^^lo+o/l ^ia« • for this class typical presents the full evolution of that multiple heart, which is developed (so to speak) out of the principal dorsal trunk usually found m the lower Articulata : whilst it also exhibits that uniformity in its structure, and in the distribution of the vessels proceeding from it, which are obscured in the higher Articulata by the unequal development of the successive ° , -. .-i -t__x* .r^ rt «+;^lov. A^Qna This p.hnnpre ot segments, and by the specialization of particular organs. This change of tynpe, however, is by no means abrupt. It appears from the researches of Mr Newport,* to whom we owe the greater part of our knowledge ot lulidm Myriapods necung iiiik utjuw^eii u±xc _ly^ j x xc*^^ —~ — — o — % < of the dorsal vessel are very thin, and the valvular constrictions between its successive segments, which are formed by reduplications of the mus- cular tunics, are by no means complete ; further, the number of these with that of the movable segments A cnamuers, wxiiun uui x c^jj^xx^o y»i^ wim,v v * ™~ — ± a body, is very large, being no less in one genus than seventy-five, greater multiplication even than this, is seen at an early period in the life of the Iulid* ; for each of their movable segments, to which two pairs of legs are attached, is formed by the coalescence of two original segments; and every one of the latter at first possesses its otaroMm- sion of the dorsal trunk, the indications of which are seen, even m the adult animal, in the duplication of the cardiac muscles and of the arterial trunks on each side, whilst the cavities have completely coalesced. It Mr. Newport and Dr. T. Williams larva of lulus, the visceral cavity contains a corpusculated fluid ; and oscillatory This Ilu-Lti iiiciy uc o^v/ij- wv j-rx-/* ^.w^. ~~ — — ^ , v before the pulsations of the dorsal vessel can be detected, and before the tracheal system is developed.— In some of the Geophtiidce, the number of distinct chambers is even greater than in the Iulkke, being no fewer hund sents a higher type of structure. * See his Memoir ' On the Nervous and Circulatory Systems of Myriapoda and Macrourous Arachnida, ' in " Philos. Transact.," 1843. I l 236 OF THE CIECULATION OF NUTEITIVE FLUID. i • ! 225. In the Scolopendridce, on the other hand, the number of cham- bers is reduced, in accordance with that of the segments of the body, Fig. 116. \ 1 -- htt * r>* 10 - 12 '£ i *^_ n 1 > 15 i ^-9; f+ '-; JS ' being never greater than twenty- one (Fig. 1 1 6), and sometimes as small as fifteen; the muscular portion of their walls is much more developed ; and the valvu- lar partitions which isolate the successive segments from each other, as well as the valves which guard the orifices of the vessels, are much more complete than in the lower tribes. The an- terior and posterior portions of this dorsal vessel, and the parts in immediate connection with it, are shown in Fig. 117; in which the figures 1, 2, indicate its first and second chambers, and 17, 18, 19, 20, 21, those of the corresponding segments at the opposite extremity. The walls are formed of two layers of muscular fibres, some of them annular and others longitudinal ; and similar fibres may be traced upon the principal systemic ar- >K teries. The of these %&?[ fibres Ak Dorsal vessel of 8co- lopendra, composed of twenty-one segments, with its alar fibres. purpose is obviously to produce contraction of the chambers; their dilatation being accom- plished by the bands of alar fibres, f, f, which extend to a considerable distance from either side of the dorsal vessel, and are inserted into the dermo-skeleton of their own segment. Each chamber has a pair of apertures guarded by valves, which is probably for the entrance of venous blood ; but the source from which they receive it has not been clearly made out; it is probably, however, the great sinus formed by the pericardial sac which surrounds the dorsal vessel. From each chamber, also, a pair of sys- temic arteries, A, h, is given off, which are espe- cially distributed to the organs of the upper side of their own segment, but inosculate with those Fig. 117. • - j of other segments. From the most anterior Circulating Apparatus of Sco- lopendra:—A, cephalic segment; B, basilar segment ; c, foot-jaws ; 1, 2, 3, anterior cardiac chambers ; 17, 18, 19, 20, 21, cardiac cham- bers of the posterior extremity of the body; a, antenna ; b, c, cephalic ganglia ; d, eyes ; e, ex- tremity of foot-jaws ; /, f, alar muscles; r/,g, valvular partitions ; h, h, systemic arteries ; i, Jc, I, m, n, their subdivisions, some of them inosculating with the hepa- tic vessels o ; p,p, aortic arches ; q, median trunk, continued from the dorsal vessel ; r, r, aortic arches reuniting, to form the ventral trunk ; s, mandibular ar- tery; t, cephalic artery; u, v, secondary arches ; w, anterior continuation of the ventral trunk x, y; z, nutrient arteries of the dorsal vessel. chamber (1) is given off a pair of large arches, p r, p 7% which encircle the oesophagus, to meet again upon its under-side in the ventral trunk, xy; but it also sends forwards a median trunk CIKCULATION IN MYRIAPODA 237 q, that gives off two smaller pairs or arches, u v which in llk ° ^ r meet in the anterior continuation of the ventral trunk, ^ From t e median trunk are also given off the vessels which supply the , eephahc segment and its organs of sensation j the lateral branches, s t which supply the mouth and its appendages, being furnished by a trunk that comes off from the principal aortic arch on each side. The distribution of the arte- rial branches of the dorsal vessel is extremely minute ; even its own parietes being furnished with distinct nutrient branches,*. _ The ventral trunk which lies upon the ganglionic cord, and is principally formed asms yarded as garuea as representing the < aorta' of Yertebrated animals ; it is ol large diameter in the anterior portion of the body, but gives off a pair of arterial branches in each segment; but ^J^we^t? £. reduced to a comparatively small size, there subdividing into the llgs 7 : but one set of them is distributed to the trachea j and it is worthv of note that the trachea and blood-vessels are mixed-up together in a remarkably intricate manner in the peritoneal membrane.-The course of the blood has not been fully made out ; hut there is a strong- probability from analogy that the venous system is altogether lacunar ; and that the blood which has circulated through the system together with that which has been sent to the tracheae for aeration, finds its way to the great pericardiac sinus, and thence re-enters the dorsal vessel through the apertures in its successive chambers. A portion of this mixed fluid will be at once sent outwards by the contraction of each chamber, into the arterial trunks proceeding from it; but the principal portion will be transmitted from behind forwards by the peristaltic action of the chambers, those of the anterior segments dilating whilst those ol the posterior contract ; and thus the current wi 1 be directed through the aortic arches into the ventral trunk, and also through the arterial branches supplying the head. Besides the dorsal and ventral trunks, we find a pair of lateral trunks, o, which are specially connected with the hepatic organs ; the branches of these inosculate with those of the ventral trunk j but in what way the blood is propelled through them is un- known—In the family of Bcutigeridm, an interesting transition .* pre- sented to the structure which the dorsal vessel possesses m .Insect s , foi everv alternate chamber is much smaller and shorter than the one before and belind it, and receives very little blood from its auricular orffices, so that, the total number of chambers being sixteen the number of the prin- cipal chambers is no more than eight ; whilst at the same time, the whole organ is shorter and of more compact form. %96 It is not a little remarkable that Insects should have been long regarded as unpossessed of a proper circulation ; the peculiar provision fo? the conveyance of air through the interior of their bodies having been supposed to render the movement of blood ^necessary *™*> however is by no means the case ) for the circulation m Insects is at least as active as it is in other Articulata ; and it is only such an extra- ordinar^ rapidity in the flow of blood, as would have been required by S&SS^ondarfdl energy, if their respiration had been localised in one organ, which is rendered unnecessary by the universal diffusion \ !. i ■ i 238 OF THE CIRCULATION OF NUTRITIVE FLUTD. J of that function.— We find in Insects a dorsal vessel (Fig. 57, a a), formed upon the same plan as that of Myriapoda ; but the division into chambers is restricted to the abdominal portion of the body, the vessel being continued through the thorax to the head, as a simple contractile trunk Thus it happens that the number of distinct chambers never exceeds eight ; and it appears that in many Insects it is even less. The muscular walls of these chambers are considerably developed, and their valvulai My The dorsal vessel is surrounded, as move; this may, therefore, be regarded as a venous sinus, from which the blood is received into the several chambers, by a pair of valvular orifices within each. The fluid is propelled from behind forwards, through the successive chambers; and is then driven onwards to the head, through the trunk which is the anterior continuation of them. Several branches have been detected, into which this subdivides for the supply of blood to the parts of the head : but no pair of aortic arches for the conveyance of blood to the under side of the body, has yet been made out, although a ventral trunk has been discovered by Mr. Newport, lying upon the gangliated nervous column, as in Myriapoda. The course of the circulation has been chiefly watched in transparent Larvae, and m Pupae during their development; and it has appeared as if, when the currents had once passed out of the dorsal vessel, they made their way rather through lacunae among the tissues, than through distinct vessels. The principal stream of blood from before backwards does not seem to flow along the ventral trunk, but through two lateral pass*™* which lie near the ventral surface ; and it is from these that the se^- dary currents diverge, which pass into the wings and legs, and then return back to the mam stream. These currents, too, seem to be rather lacunar, than restrained by distinct parietes ; but it is probable that the walls of the passages may be more complete in the perfect Insect.— The mode m which the aeration of the blood is provided-for in Insects, was long misapprehended ; but from the enquiries of M. Blanchard it appears that the tracheae are ensheathed, even to their minutest ramifi- cations, by prolongations of the sanguiferous canals, and that the blood is therefore aerated wherever the tracheae penetrate.* This view is in perfect accordance with the fact long since observed by Mr. Bower- bankt and others, that the current of blood in the ' nerves' of the win^s moves in a space which completely surrounds the tracheae. So far then, as the course of the circulation in Insects is yet known, it may be probably considered to be as follows. The blood impelled forwards by the dorsal vessel, is transmitted to different parts of the body, either by distinct vessels, or by lacunae ; after passing through the tissues, it a V ^°t i 8 Sci \ Na V!.f S6r -' Zool > Tom - ix -> xii - Such > at least > a PPear S to the Author to be the most probable interpretation of the facts ascertained by M. Blanchard's in- jections, taken in connection with those observed in the living Animal.— The statements of M Blanchard have been called in question by several Anatomists, more especially by M Jolly (Op cit. Tom. xn.); they have been confirmed, however, by others (Op. cit., Tom. xv See also the observations of Dr. T. Williams (" Ann. of Nat. Hist.," Vol. xiii p 194) who states that those minute ramifications of the trachese, in which the spiral fibre dis- appears (§ 302) are not accompanied by blood-channels. f "Entomological Magazine," April, 1833, and October, 1836. CIRCULATION IN INSECTS. 239 finds its way by imbibition into the outer sheaths of the smaller tracheae, which thus serve to collect it for aeration ; and from these it is trans- mitted, whilst undergoing exposure to the air contained in the tracheae, towards their external orifices, where it is collected from their sheaths by a system of canals which convey it back into the great pericardiac sinus, whence it enters the dorsal vessel. structure than is that of the Imago; being sometimes almost as destitut e of valvular partitions as is that of the Annelida, and very commonly pre- ler a development than does that of the lower Myriapoda. But in its advance towards the perfect state, a gradual thickening of its walls, and a completion of its valvular partitions, may be noticed; and at the same time the whole organ becomes contracted in length, and presents a more concentrated condition. In many aquatic larvae, espe- cially of the order Neuroptera, there are leaf-like appendages affixed to the tail, in which the circulation may be distinctly seen, the streams passing-off in loops from the main trunks, and entering them again, so as to*be conducted to the posterior extremity of the dorsal vessel. Previously to the metamorphosis, the currents cease in these organs ; win observed which very seldom continue for any length of time after the last meta- morphosis, although they may be frequently seen in individuals that have recently emerged. win is obviously the cause of their complete deficiency in reparative power ; no losses of substance in them being ever made good, so that old bees may always be distinguished from young ones by the chipped indented edges of these organs, resulting from the accidental injuries to which they have been subjected. — In certain aquatic larvae, especially of the Gnat tribe, in which the visceral cavity occupies a large part of the body, this may be seen to be in the freest communication with the dorsal vessel, which has not itself any vascular prolongations ; and the move- ment of the blood, which can be easily watched, on account of the multitude of corpuscles which it contains and the transparency of the bodies of these larvae, appears to take place from behind forwards in the dorsal vessel, and from before backwards in the great venous sinus, without any diverging currents ; not only the parietes of the body, but all their contents, being in such immediate relation with the circulating fluid, that no further provision is necessary. In some larvae whose deve- lopment is yet less advanced, even the dorsal vessel appears to be wanting, although the fluid of the visceral cavity is in a state of con- tinual oscillatory movement. 228. The Circulating apparatus of the Arachnida presents us, at least in the higher forms of the class, with a much greater completeness than that of Insects ; and this is especially seen in the addition of a set of vessels that is specially subservient to the aeration of the blood, in accord- ance with the oreater localisation of the respiratory organs themselves. Nevertheless there is no essential departure from the plan of structure Myriapod Macrour such as the Scorpion, the conformity in the arrangement of the vascular t l ■ K 240 OF THE CIRCULATION OF NUTRITIVE FLUID. M system is extremely close. The following are the most important of the Mi Fig. 118. fl> 711 I Diagram of the Circulatory and other organs in Butlncs ; —a, antennal claws; B, eyes; c, c, pulmono-branchiae ; d, alimentary canal; E, anal orifice; F, poison -glands ; a, anterior continuation of the dorsal trunk, giving off 1, 2, 2*, branches to the posterior legs, besides the two great arches which form the ventral trunk; b, cephalic ganglia; c, optic nerve ; d, great subcesophageal ganglion ; e, ocelli ; f t supra-cesophageal artery, giving off 6 — 14 branches to the cephalic ganglia and organs of sense; g, lateral trunks of subcesophageal arteries, giving off 3, 4, 5, branches to the anterior members ; k, h, chambers of the heart, 1 — 7 ; i, i, ventral artery; k, &, branches given off in each segment ; I, I, branches of caudal artery; m, m, ganglionic cord; n, n, portal system of vessels; o, o, its branches; p, p, systemic arteries given off from the cardiac chambers ; q, q, great caudal artery; r, r, its branches of communication with the portal trunk s, s; t, termination of cardiac portion in anterior dorsal trunk ; u, u, auricular openings into cardiac chambers; #, diaphragm dividing cephalo -thorax from abdomen; y, anterior pair of systemic arteries dis- tributed on this ; z } visceral arteries. into the Circulating appa- ratus of the Scorpionidce. — The dorsal vessel runs continuously from the pos- terior extremity of the tail as far as the cephalo-thorax, and may be described as consisting of a cardiac por- tion (Fig. 118, h,h, 7) which occupies the abdo- men, a dorsal artery, a, which is the anterior con- tinuation of this, and a caudal artery, g, g, which is its posterior portion. The structure of the cardiac portion, which contains eight chambers (the pos- terior one, however, being very imperfect), is similar in most respects to that of the dorsal vessel of th< higher Myriapods and per- fect Insects ; but it differs in this, that the valvular partitions between them are much less complete ; a difference which j3robably has reference to the fact, that the cardiac portion has to send a great arterial trunk backwards as forwards, enters the cardiac cham- bers from the surrounding through the auricu- u; and a as well The blood sinus, «rv^ lar orifices u portion of it is sent forth without being pro- agam, pelled into the adjacent chambers, through the sys- temic arteries p 9 p. The anterior continuation, or dorsal trunk a, passes through the septum x, which divides the cephalo- thorax from the abdomen ; and soon afterwards gives off a pair of large branches, which pass round the oesophagus, like the aortic ■ CIRCULATION IN MYBIAPODA 241 Myriapoda gives off large branches; i, 2, to the two posterior pairs of legs, and a smaller one 2* to the thorax ; after which it separates into three principal trunks, of which one, /, is median, and runs above the oesophagus, giving oil branches (6-14) to the cephalic ganglia and organs of sense, whilst the ophagu two remaining pairs of legs (3, 4) and the great prehensile claws (5). ™™+ +v*oy*o ia * vptv marked conformity to the type ottli In this § penura, i« **u h Besides these vessels, the dorsal trunk gives off proper visceral branches, z, which proceed backwards along the anterior portion of the alimentary canal, inosculating with branches from the systemic arteries p p The posterior continuation of the multiple heart, constituting the caudal artery g, g, passes backwards to the extremity of the tail giving off inlts coursed ^stemic arteries I, I, and also the latera branches ,, r which communicate with the portal trunk • ..-The ventral trunk ys which lL upon the upper surface of the gangliated nervous cord, extends Lckwards from its commencement in the aortic arches, nearly to the termfrTation of the tail ; gradually diminishing m diameter, as it gives off minute vessels for the nutrition of the cord, and successive pairs of branches, *, k, which pass downwards to coinmunicate with the portal trunk with the terminal nerves proceeding from the ganglionic cord. Beneath Mr tile efaMllOUIU UUlUiim yyu iinv AW xxv,v.w. , o y uf„,l as the portal ; the purpose of which appears to be, to collect the blood from the systemic vessels, and to transmit it for aeration to the pulmonary branchiae, c, c. This trunk is formed by the coa escence of ^^°£ various sources; but especially from the ventral trunk m the aMomrnal region, and from the caudal artery in the tail, **^^™£™** entirely distributed upon the respiratory organs. -Such is the general distribution of the proper < vascular' portion of the circulating W^atus, which seems to be altogether arterial in its character; the venous circu- lation in the body at large would seem to be altogether < lacunar win 1st the return of the blood from the pulmonary branchiae to the h«u*g- bably takes place by definite canals.-The course of the Wood, ^e^ would seem to be as follows. Of that which is "^^^^ atory organs to the chambers of the multiple heart, one part is sent .forth Te cephalo-thorax and its organs of sensation and motion; whilst a por- tion of it is carried backwards through the aortic arches and ventral trunk partly for the nutrition of the ganglionic cord and partly for trans- Xion into the portal system. So, again, the caudal trunk conveys the S I WVwards to the extremity of the tail, giving off nutrient Wh^oTetrLs pa* of that organ and also — £g a pa* of its contents directly into the portal system. The great portal trunk, (wMch mS perhaps be considered as made up by the coa escence of the +3. TwJrLals that exist in Insects and many Articulata) receiving trodtom to sources, and collecting that which has been distributed through the tissues by the systemic branches of the dorsal and ventral I I It I 1 242 OF THE CIRCULATION OF NUTRITIVE FLUID. Fig. 119. trunks, transmits this fluid for aeration to the pulmonary "branchiae, from which it is returned by a set of ' branchio-cardiae canals ' to the great cavity surrounding the heart. 229. The circulation has not been studied with the same minuteness in the Araneidce; but from the researches of M. Blanchard* it appears that the course of the blood is essentially the same as in Insects, but that it is distributed by more perfect vessels. The dorsal vessel forms a multiple heart of four or five chambers in the abdominal region (Fig. 119); but the partitions be- tween these chambers are often scarcely per- ceptible, so that the cardiac cavity is really single but elongated, as in the Stomapod Crustacea (§ 230). The blood is sent forth from this organ, partly by systemic vessels directly proceeding from its segments, but chiefly by the dorsal trunk (a) which forms its anterior continuation ; and from this, on its entrance into the cephalo-thorax, nume- rous branches are given off to the various organs of sensation and motion, and also to the viscera contained in that division of the The blood thus distributed through the system is stated by M. Blanchard to find its way to the pulmonary branchiae by lacunar passages, neither ventral trunk nor a portal system of vessels having been yet made out ; and from the respiratory organs it is carried back to the heart by distinct branchio-cardiae canals (6, c,), which discharge themselves into the several body. Heart of My gale:— a, arterial trunk proceeding to cephalo-thorax ; h, ves- sels of the anterior, and c, vessels of posterior pulmonic apparatus. chambers of the multiple heart. The more diffused the respiratory function is rendered, by the prolongation of the pulmonary branchiae into tracheae, the more does the circulation resemble that of Insects in the predominance of the lacunar over the vascular type. 230. It is among the Crustacea, that we find the sanguiferous system presenting the most developed form under which it exists in the Articu- lated series. In the lower orders, however, the segments of whose bodies are nearly alike throughout, the contractile portion of the dorsal vessel is elongated, and the distribution of its branches is nearly uniform in each segment ; but the advance towards a higher type is shown in the order Stomapoda, the members of which have usually an elongated fusiform heart, developed as a muscular dilatation of a part of the dorsal vessel, which, towards its anterior and posterior extremities, possesses the cha- racter of a sanguiferous trunk. The same is the case also in the Limulus. But in the order Decapoda, which includes the most elevated forms of this class, we find the heart contracted into a short fleshy sac, possessed of considerable muscular power, and concentrating in itself the propellent force which is diffused in the lower tribes through a large part or the whole length of the dorsal trunk. This organ in the Lobster is situated on * "Annates des Sciences Naturelles :" 3 e Ser., Zool., Tom. xii., p. 317. i CIRCULATION IN CRUSTACEA. 243 * the median line, at the posterior part of the cephalo-thorax (Fig. 120, a, d); from its anterior part is given-off a large cephalic trunk (e) which passes Fig. 120, n c Circulating Apparatus of Lobster ;— A, Heart and Systemic Arteries as seen from above ;-a, smaller antennae ; b, larger antennae ; c eyes ; d, heart ; e, ophthalmic artery; /, antennar arteries 5 a, A, superior abdominal artery. B ; Great Ventral Sinus, receiving venous blood from the system, and trans- mitting it to branchiae ;—a, first pair of legs (claws) ; b, venous sinus. c, Respiratory Circulation, as seen in a transverse section ol one of the segments :— a, leg ; b, venous sinus ; c, branchio-cardiac trunks ; d, branchiae ; e, branchial veins, or efferent vessels, uniting to form branchio-cardiac trunks ; f, branchial arteries proceeding from venous sinus. ■ forwards, and soon subdivides into branches for the supply of the eyes and neighbouring organs, and also a pair of antennary arteries ; whilst from its posterior extremity is given-off the abdominal or caudal artery (g h) from which successive pairs of branches are sent-forth laterally to the segments of the abdomen. The same arrangement prevails in the Crab (Fio\ 58) ; but the posterior trunk is there much smaller in propor- tion in accordance with the undeveloped state of the abdominal seg- ments These two trunks, with the heart, obviously represent the entire r2 i * 1 '*- li I 244 OF THE CIRCULATION OF NUTRITIVE FLUID. dorsal vessel of the lower Articulata ; the transitional form being shown in the Scorpion. Besides these trunks, a pair of large hepatic arteries is given-off from the sides of the heart, which are exclusively distributed to the liver ; whilst beneath the caudal artery, a large sternal trunk origi- nates, which bends down towards the ventral aspect of the body, where it divides into an anterior and a posterior branch, the former of which supplies the thoracic members, whilst the latter distributes blood to the under surface of the abdomen. This may probably be regarded as homologous with the ventral trunk of the Scorpion, though it has not yet been shown to have any connection with the cephalic by aortic arches. The blood conveyed to the body by these arteries, appears to return through the lacunae of the tissues into two sets of large venous sinuses ; of which one consists of a series of flattened cavities, freely communicat- ing with each other, which lies immediately beneath the shell of the back, covering the upper surface of the heart and dorsal trunks, and ob- viously representing the pericardiac sinus which incloses the dorsal vessel in other Articulata ; whilst the other series (&, Fig. 120, b, c) lies at the bases of the branchiae, on each side of the inferior surface of the thorax. The former collects the venous blood from the dorsal and caudal portions of the body, and carries it back to the heart, which it enters by two pairs of orifices, guarded by semilunar valves. The latter (which apparently corresponds to the 6 portal' trunk of the Scorpion) collects the venous blood from the maxillae and legs, and distributes it by branchial arteries (c, f) to the branchiae (d) for aeration. From these organs it is again collected by veins (e), which all coalesce in a pair of large trunks, the branchio-cardiae canals (c), that convey the aerated blood back to the heart. Hence, in that central organ, the venous blood received from a portion of the body is mingled with the arterial blood that is transmitted direct from the gills ; and the fluid which is propelled through the systemic arteries is hence of a mixed character, as in .Reptiles, — a class to which the Crustacea have many points of analogy. Thus the localisation of the respiratory organs in the higher Crustacea, as in Arachnida, involves the existence of a special respiratory circulation. 231. In the lower orders, however, the blood is aerated in its progress through the general system, as in Insects; and in many of them, the arterial as well as the venous portion of the system appears to be alto- gether i lacunar.' In one of the most degraded forms of the class, we revert to the simplest possible type of the Circulating apparatus ; even the dorsal vessel, which is so characteristic of the Articulata, being defi- cient in the Pycnogonidce (Fig. 105). In these curious animals, there is not the least trace of any vascular system ; but the visceral cavity of the body and limbs, that intervenes between the integument with its mus- cular lining and the stomach with its prolonged caeca, is occupied by a corpusculated fluid, which is kept in a state of continual flux and reflux, not only by the general movements of the body and limbs, but by the peristaltic contractions of the walls of the digestive sac. For when the contraction of the central cavity forces a part of its contents into the gastric caecum of either of the legs, that caecum, by its dilatation, presses out a part of the circumambient liquid, which will flow into the cavity of the thorax, whence it may pass into that of any other limb in which space is formed to receive it by the contraction of its gastric caecum. CIRCULATION IN MOLLUSCA, 245 There is no special organ of respiration ; so that the aeration of this fluid ordinary imperfect circulation, must be ' sufficient for the wants of these inert ail 232 S *From the view which has thus been taken of the Circulating ap- paratus in the higher parts of the Articled series, it will be seen that, notwithstanding the diversity of detail, there is a very general conformity to a definite plan ; and that the tendency, as we ascend from below up- wards is to a concentration or specialization in a single organ of that impulsive power which, in the lower tribes, was diffused through various parts of the system. Now when we follow a similar course with regard to the Mollusca, it will be seen that m almost the lowest forms of that series, the central organ is as powerful, and the circulation ^ much carried-on through distinct vessels, as m the highest Crustacea. The explanation of this general inferiority of the circulating system m the Articulated series is partly to be found, as we have already seen, in the tneml dTffuston of the respiratory apparatus; but it seems to be m part o ejected with the mechanical arrangements of the body, the continual movements of whose several parts furnish an additional means of propul- sion to the blood which is meandering through their lacunar spaces and cavities Even in Vertebrated animals, we find that the general acts oi locomotion have an important share in promoting the flow of blood, especially through the venous system ; its trunks being allowed to fill, and being then forced to empty themselves towards the heart (any reflux being prevented by their valves), by the alternate relaxations and con- tractions of the muscles which are so situated as to press upon them On the other hand, in the comparatively inert bodies of the Mollusca, the blood would stagnate in its course for want of such assistance, if it were not kept in motion by a powerful force-pump, in the form of a compact heart, with firm muscular walls.— In most of the instances in which we have hitherto found an organ of propulsion materially affecting the current of the circulation, it has transmitted the blood which it has received from the venous sinuses or canals, into the principal systemic arteries,' and may therefore be designated as the systemic ventre Among the Annelida, however, the impelling cavities are frequently situated at the commencement of the branchial ^ ve % els ' a £^e ^ ^ considered as representing the pulmonary ventricle of ^r animals In the Mollusca generally, we find, superadded to the ™*™£aj ™* or contractile cavity, adapted to receive the blood transmitted to the heart, and to propel it into the ventricle j and the existence of these two cavities constitutes the typical character of the heart through nearly the whole of that sub-kingdom, although many variations are presented m their form and situation. Notwithstanding this higher development of the heart, the rest of the circulatory apparatus remains in a state of relative incompleteness; for in the lower classes of this series the distribution of blood to the general system is almost entirely eflected by lacunar spaces, of which the visceral cavity forms part, it being only m the respiratory organs that it moves through distinct vessels ; > and even in the higher tribes, in which the arterial part of the systemic circula- tion is generally truly vascular throughout, the venous portion of it is still in some degree lacunar. ■ 1 1 1 1 1 1 4 ■ ■ 1 246 OF THE CIRCULATION OF NUTRITIVE FLUID. Fig. 121. 233. In the Bryozoa there is no other circulation than the flux and reflux of the nutritive fluid, which has transuded through the walls of the alimentary canal into the visceral cavity, whence it passes into the ten- tacula;^ a continual movement being kept-up by the peristaltic contractions of the alimentary canal (as in the Pycnogonidse, § 231), and by the altera- tions in the form of the visceral sac, consequent upon the retraction or projection of their polypoid bodies. The visceral cavities of the several < zooids that have originated by gemmation from the same stock, seem to retain their connection with each w \ a A ^np= /, B / ,t»i other, although this is sometimes narrowed and prolonged so as to form but a slender tube, stony walls of the ' cells' which invest the The soft n r \ ) (f- bodies of many species of Uschara, Lepralia, &c, are marked with punctations, which are in reality the orifices of short passages extending into them from their internal cavity; and these passages are occupied by prolongations of the visceral sac, which thus convey the nutrient fluid into the substance of the framework formed by the aggregation of the calcified tunics of these animals. 234. Although, in the Tunicata, we find a special for the regular circulation of n — ^ ■ / .»- W \ provision _ nutritive fluid, yet this is not very far removed from the simpler arrangement which suflices in the Bryozoa ; for the system of passages in which that fluid moves, might be considered as an offset from the general cavity of the body, which still forms an important part of the circuit, find a distinct heart, usually of a somewhat We a elongated form, . ai id generally situated neighbourhood of the ovarium ; it the m 7 _ _ 7 however, merely a portion of the sinus-system furnished with muscular parietes, and it is not yet divided into auricular and ventricular cavities. In the long-bodied Polyclinians, the heart is situated at the extremity of the post-abdomen (Fig. 121, o); but in the Botryllians (Fig. 51), which have no proper abdominal cavity, it is brought into much closer approximation with the branchial sac ; and in the Salpce it holds nearly the same relative posi- tion (Fig.^ 122, A, e). In the solitary Ascklians, however, it usually lies between the branchial sac and the ventral surface of the body, partly exca- vated in the muscular tunic. Pelonaia,* — which in its general form and in some parts of its structure exhibits an obvious approxi- mation to the Sipunculida, as also (in common with that group) to the Annulose type,— there * See Messrs. Forbes and Goodsir, in "Edinb. New Philos. Journ," Vol. xxxi. p. 29. Anatomy of Amaroueium prolifemm: — A, thorax; B, abdomen; c,post-abdomen; — c, oral orifice; e, branchial sac ; f, thoracic sinus ; i, anal orifice; i', projection overhanging it; j, nervous ganglion ; k, oesophagus ; I, stomach surrounded by biliary tubuli ; m, intestine ; n, termination of intestine in cloaca; o, heart; o', peri- cardium; p, ovarium; p', egg ready to escape ; q, testis ; r, spermatic canal ; r', termination of this canal in the cloaca. In the curious genus CIRCULATION IN TUNICATA 247 seems to be no heart, although the vascular portion of the ^culatmg a^ratus is more complete than usual ; but it is not ^^^ doLl trunk is contractile throughout and supplies the place of that 01 ^gan The blood does not ordinarily seem to move, m any part ot its course, through proper vessels with distinct parietes; but flows through a system of SL, which are in free communication with the general cavity of the bortnd which occupy the whole space between the second and third tunfc's save where these are adherent to each other. There is usually a Xcipal sinus on the dorsal, and another on the ventral aspect of the Todv and these communicate with each other by an intermediate net ; w ik ofdZU, which are very close and minute m some tribes but Inch wkta apart as well as larger, in others. From this sinus-system, mucn wiuex ^ , species m which this enve- § The most curious feature in the action of Z %££*£%&*** thisgroup, is the ..alternation -which presents itself in S direction of the flow of blood j for this renders it impossible to desilate either set of vessels or passages as arterial or venous since both are Arterial, and both venous, in their turns. The contraction of the heart in al the species which are sufficiently translucent to allow it to bHbserved during life, is somewhat peristaltic m its character V prooeed- S ff W one ^mit^ of its cavity towards the other j after the pulsa- tionrhave continued in either direction for a minute or two, a short nXm^ntSs place, during which the current of ^™%££ stand • and the peristaltic contraction then recommences in the opposite Erection the flow of blood being now first directed towards those parts W which it had last returned. If the course of the circulation be waShld in an individual of *™~ ^^ *™ «,— envelope it will be seen that the blood when propelled towards the Wx! passes through the space left between the viscera ^ and the lining membrane of the cavity; chiefly, however, along either i do «al or ^ ventral asnect When the ascending current takes the latter direction (wlS ^ coTefponds to its course ^^^ft^^4S vertical canal, which is desigBated as the great ' thoracic £J^«* sitmq From this sinus, a great number of vessels pass off transveiseiy Tund EtSlSl sac; and these are ^^^Zl^T^ communicating ££*%£* ^ZZl^U^t^Z "fa " a minute network is thus tormea, m W1JJ ^ ^H-wnrk- flip apratprl tion to the water that enters the sac. From this network the aerated blood is collected into the < dorsal' sinus, which also receives blood that has been transmitted to it direct from the thoracic sinus by means of a ve se ^ that passes round the branchial orifice ; so that he fluid which it vessel tilery ^ 9ji ^^w From the dorsal sinus the blood When reversal of te elation takes place, the blood is first transmitted reversal ot tne ^ ^^ ^ & t ls d Sitl branchial sae ; and it finally returns to the heart by th 2oT Thettoulation in all the Tunicata is performed upon a plan essentially the same ; bat its peculiarity in certain compound OmMme I" ; B I ! 248 OF THE CIRCULATION OF NUTRITIVE FLUID. deserves a special notice. In the Perophora (Fig. 138), the several indi- viduals are not included in a common < test,' but grow by footstalks from a common < stolon.' Each of these stalks contains a double channel for the passage of blood, and these channels communicate in the common stem with those proceeding from other individuals ; so that a vascular communication exists among them all, and this extends also to the unde- veloped buds which sprout forth from the stolons. One of the channels m each peduncle enters the heart of the animal which it supports ; and of the blood thus received into the propelling cavity, a great part is transmitted along the ventral canal over the branchial surface, whilst the remainder is distributed to the visceral apparatus and the mantle. Both these currents reunite in the dorsal sinus, which then conducts the blood, not back to the heart again, but into the peduncle, in which it may be seen to flow towards the principal stem. As in other cases, how- ever, a reversal takes place about every two minutes ; the blood' then flowing towards the body in the peduncular channel that communicates with the dorsal sinus, and returning from it in that which is continuous with the heart. When one of the animals is separated from its peduncle, the circulation is at first disturbed, but soon regains its usual regularity; a new communication being apparently formed, by which a free passage of blood takes place between the dorsal sinus and the heart.* In many of the Salpidce, we find a much more complete system of Fig. 122. B m^^-ri^o^T^r' aS Se T fr ° m the *% at A > and frOT1 the antral vascular passages, adapted to diffuse the blood over the membrane linino- the general cavity, as well as upon the special respiratory organ. From the heart (Fig. 122, A, e) proceeds the ventral sinus (/), which passes * See Mr. Lister's Memoir in the " Philosophical Transactions," 1834. CIRCULATION IN BRACHIOPODA. 249 towards the anterior extremity of the body, giving off n^ta^ aTright angles on either side, from which arise numerous small er ra As the whole forming a minute net-work over the whole of the dilated S cavity (b) : whilst a separate set of channels distributes blood jTS^^^toBOOB fold "which proj ects into its lintermr From all these vessels it is collected into the dorsal smus, which conveys it back t the he!" It is also transmitted through the lacunar spaces jhich immediately surround the viscera. The same alternation may be wit- ZZdin the direction of the flow, in the Salpse, as m other Tunicata but herT as elsewhere, it has been noticed that what may be considered afth £e /current,-*, e. the forward movement of the blood from the heart alont the < ventral' trunk,-is of much longer duration than the levTr'seS* ffi the blood is propelled from the heart through the dorsal trunk ; the number of pulsations being usually about twice as great for ' th \Tr^ r B^Tuollu^ we find a more complete system of arterial vessels and the heart is divided into an auricular and a ventricular cfvltv but the venous circulation is carried-on as m the Tumcata by a svltem of sinuses in free communication with the ge ner al ca yity of the syspeiiiux _-u:^ QVA vATnartaHe for their bilateral body, y-mmetry Brachiopoda SL plane, there are two separate and distinct hearts, one for either Z\i oT the body. Each has a feebly-muscular ventricle, which propels the blood it receives from the auricle into two arterial trunks ; the larger ofwhich ItXtes it to the two halves of ^™^«^^ ventricle, the smaller to the viscera and muscles. The pallial artery terminate at the margin of the mantle m the ™ G ™%^™*J e ^ from which the blood is returned by large sinuses m the ™*^™*J™ mantle, that discharges it into a large common sinus at the back : part ot the visceral chambe?. This sinus also receives, from the visceral ^cavity and its extensions, the blood which has been distributed among the vLera and muscles, by the second arterial trunk proceeding from the ventricle ; and it discharges its contents into the auricles of the two hearts, each of which has a wide gaping orifice for its admission. The * The account of the Circulating aPP^^-^^^SSfeae^^ Prof. Owen's description of the < Anatomy ; rf the Ty^^J^^Mahed hy the to Mr. Davidson's Monograph on the 'British Fossil ™f ^^ le ^ him to the con- Paheontographical Society, 1868 .-The Authors ^ ^^^ uMapoillk has not elusion, that one very remarkable extension of Jhe sinus sy .tern £ been noticed by Prof. Owen. The ^"^^^.£ e '3£' »% * ^ ™^> ^ the valves, and which is usuall y rega f ded as c ^utm toe ^ of the ' sM1 onefold of the mant e ; another f ? + ld ,^!." /^ a teX dissolving the calcareous matter „ not to be ^^^^^^^S^S^^ti. to" this adhesion that the ^^rfdl^Si^^««Med" 'the'mantle' from the shell, long since ?,2U Prof Owen, is really due ; but wherever they do not adhere, the space between ^ 0t 1« to be occupied by blood, which thus fills a capacious sinus-system between the them seems to be occjeat,y , ^ & tion o£ the Brachiopoda, two folds of th ^™ B V C e S ses which penetrate the substance of the shell This smus- those ^^^^JfJ^SmparaWte to that which intervenes between the second and system appears to be , str cuy p ^^ ^ ^te into the *W TSliotds Respond with those which extend into the 'test oi that class. ttZ ^^7^ou™ oi these, in his description of ' The Intimate Structure of the IheUs oi ^CchLpodaTforming p'art of the Introduction to Mr. Davidson's Monograph already cited (p. 30.) I l\ ! ■ 250 OF THE CIRCULATION OF NUTRITIVE FLUID. pallial portion of the circulation is obviously respiratory, since the blood spread over the extended surface of the mantle is more freely exposed to the aerating fluid, than is that which is distributed elsewhere; and hence the circulation of the Brachiopoda may be compared with that of Reptiles, the aerated blood which has returned from the respiratory sur- face being mingled in the central cavity with the venous blood which has returned from the system, and a mixed fluid being thus propelled through both the pallial and the visceral arteries. t Fig. 123. Pinna marina laid open, to show the arrangement of the Circulating Apparatus .—a, mouth; B, foot; c, digitiform appendage to the foot; D, byssus; e, labial tentacula; f, f, branchial those of the right side being in place, those of the left being divided near their anterior extre- mity, and turned back, so as to expose their anterior surface ; g, g, the mantle, of which the left lobe has been detached and folded back; h, posterior adductor muscle; i, first stomach, covered by the liver; K, retractor muscles of the foot; l, anus; m, glandular organ, probably urinarv; between this organ and the branchiae of the right side is seen the second stomach; a, aortic ventricle; a 1 , anterior portion of this ventricle, passing round the rectum ; b, one of the auricles turned back, the other being seen in its natural position on the opposite side of the ventricle- c, one of the branchio-cardiac canals; the other is seen in front of the adductor muscle h • d, anterior aortic trunk; e, posterior aortic trunk;/, f, pallial veins, proceeding to empty them- selves into the branchio-cardiac canals at # ; h, h, afferent vessels of the branchiae; i, canal of communication between these last, and the general lacunar system of the abdomen. 237. The chief modification of the foregoing plan in the Lamelli- CIRCULATION IN LAMELLIBRANCHIATA AND GASTEROPODA. 251 IrancMate Bivalves, consists in the development of a system of ^anchial vessels - the respiratory function of the mantle being here m great , part luperseded by the special provision made for it in the lamelliform branchiae. Although the auricle is still frequently double, there =ny hi, a s ^7e ve^ci; (Kg. U i «), of which the .alls aref ornied by macular fibres interlacing in every direction ,<£*"*£**"* into the interior. into tne interior. From this centre, which is situated between the adductor muscle and the rectum, the blood is sent by two principal arteriaUrinks, an anterior {d) and a posterior («), to the system at large ; and I then" it is returned, noVby ^^™?*^£%?Z m the substance of the foot b), of the labial teitecula (e <* &*<£ . ductor muscle (h), of the glandular organs, and of the viscera generally the contents 0} which are* collected in great part bya furro. ^ ong +1,p W margin of the mantle, into the afferent vessels (h, h), by wiiicn it s conTerd to the brands. After being exposed in these organs to the ib cunvt^cu. vv anrronndino* water, it repasses to tne fTbv two ZtS*3?lS do' not e'nter the ven- S, t r™SeTn C aurioles (6), of which one is «-£$»£*« e»ch side The branchio-oardiac canals also receiTe the Wood bi ought Tack by the venous sinuses from the general surface of the mantle, which sti 1 continues to act as a respiratory organ, although the proper branchy of the LamellibrancHate bivalves are undoubtedly the spemal *f™™*? of this function.*-Although two auricles are found m most J^dh branchiata they are not to be regarded as representing the two auricles Sr b^blnlvertebrata « 24o), of which one ™ the. bloo Urom tm^lyTwhi "m an^tner X the l^, the ventricle is SS&Wie auricle, in conformity ^*^«*£*£ the animal, and the consequent separation ol tne 238 Among the Gasteropoda, we find the same general arrangement of t e cifculatfng system; but the situation of its centre ^ and the dist i bution of its vessels, present great variation m *^\^^ sac; heart (Fig. 124, k)is composed o a ,ven ^^J^Z^r^ another. 238. the former of which is Wly pS with walls of considerable firmness, having muscular bauds ^^^^£^^0 slightly into its cavity. In ^^ZTeren\e ventricle is* partly the auricle is double ; and ma few genera, e ven f / tne aiuioi« » «™~, 7",., __ ~ through it. The blood whicn is /Ii-J ^^ ^^ distinct m these ^ S /^X S net | ork it 1 passes, not into a proper venous parietes ; and j^mt^ intercommunicating lacume, which even pour it StoThe " c ly (* H so that it bathes the external surface of * See the Memoirs of Prof. Mine-Edwards. on the ,« Circulation in << Ann des Sci. Nat.," 3* Ser., Zool., Tom. vm., p. 77. Molhisks,' in the m 252 OF THE CIRCULATION OF NUTRITIVE FLUID. the alimentary canal. From a part of this lacunar system, it is collected into the afferent trunks which distribute it to the respiratory organs * Fig. 124. Anatomy of Snail: — a, mouth; b b, foot; c, anus; d d, pulmonary sac; e, stomach, covered above by salivary glands; ff 9 intestine; g 3 liver; h, heart; i> aortic trunk; j> gastric artery; ky artery of foot; Z, hepatic artery; mm, abdominal cavity, serving also as a venous sinus ; n n, irregular canal communicating with abdominal cavity, and conveying blood to pulmonary sac; o o, pulmonary vein, returning blood from pulmonary sac to heart. whether branchial or pulmonary (d, d); and after having been aerated in these, it is returned to the auricle by the branchio-cardiac canals, or pulmonary veins (o, o). poured blood that is returned by a portion of the lacunar system, without passing through the respiratory organs ; so that the general plan of the circulation strongly resembles that which we have seen in the Crustacea (§ 230), only a part of the blood which is sent-forth from the heart, being subjected to the aerating process before returning to it again. The proportion which thus escapes aeration, however, seems to differ consi- derably in the several orders and genera of the class.— Many curious varieties in the arrangement of the vascular system of Gasteropoda might be enumerated ; but it must here suffice to mention that they princi- pally have reference to variations in the condition and position of the respiratory organs, and do not present any essential departures from the type just described. Thus in Haliotis and Patella, certain parts of the arterial system present the lacunar condition; and in Tethys, the branchio-cardiac canals unite into an immense venous sinus, which occupies the dorsal region, and which is only separated from the visceral cavity by a thin membrane.* In Doris, i again, the skin appears to act in a considerable degree as a respiratory organ ; and the blood which has been distributed to the foot and to the greater number of the viscera, is conveyed, not through the branchial circle, but through a network of vessels in the skin, before returning to the heart ; it being only the blood which has traversed the liver, kidneys, and ovaria, that is sent for * See Prof. Milne-Edwards' s account of the Circulation in these Molhisks, in the Memoirs last referred to. + See Messrs. Hancock and Embleton's Memoir < On the Anatomy of Doris ' in " Philos Transact.," 1852, p. 228. CIBCULATION IN GASTEBOPODA AND CEPHALOPODA 253 aeration aeration to the special respiratory apparatus (Fig. U3). In Eolis which has no other respiratory organ than the skin and its papillae, the Wood which has passed through the internal organs finds its way to the surface through an intermediate system of lacunae; and after being there aerated, is carried hack to the heart by branchio-cardiac canals .*— In Gasteropod Mollusks 1V1011USKS, as ill me U1 8"^ ~--~~ — , - » • ~ - developed, and supplied with blood by a large arterial trunk (Fig. 50, o). Some yet more special provision for supplying blood to the liver is not unfrequently met-with; the most remarkable at present known being m Boris, which has a peculiar contractile cavity, freely communicating with the pericardium (which seems to act as a kind of auricle to it) situated at the commencement of what may be designated the portal system of the liver Venous blood is returned into the pericardium from the body, without passing through the skin; and this, having been distributed through the liver by vessels proceeding from the < portal heart, is col- lected b Y a system of more definite veins than are seen elsewhere, and is conducted J the branchial circle, after passing through which, it is returned to the systemic heart. As m higher animals, moreover, the liver > is supplied with arterial blood by a branch of the aorta ; and this, like the blood of the portal system, is collected by the hepatic veins, and is sent to the branchial circle for aeration, t # 239 Hitherto we have usually found the respiratory organs, whether branchial or pulmonary, interposed between the capillaries of the system and the central propelling cavity ; the canals which collect the blood from the different organs of the body, uniting only to distribute it again, with- out any fresh impulse being given to their contents. The only instance of the interposition of anything like an impelling cavity between the systemic and the respiratory vessels, was seen in certain Annelida which are provided with branchial hearts (§ 222). Now in Fishes, the heart is entirely respiratory, the arterial trunk which proceeds from it being dis- tributed at once to the gills, and the blood which has been aerated in them being returned into a systemic artery or aorta whence it pro- ceeds to the body at large ; and in the class of CepJialopoda especially m the Dibranchiate order, we meet with a condition of the circulating apparatus, which manifestly establishes the transition between that of the Mollusca in general, and that which is peculiar to Fishes The systems heart of the Octopus consists of only one cavity or ventricle (Fig. 125 p), which is usually of a nearly globular form, tolerably strong and muscular and exhibits on its internal surface bundles of fibres (carnem columnw) interlacing with one another, as well as distinct valves protecting the orifices by which the blood enters it. The aorta (q), and the branches which proceed from it, distribute arterial blood to the general system ; and this is returned by means of a regular system of veins possessing dis- tinct parietes, of which one is seen at r. Of these, however, one pair * An attempt was made by M. de Quatrefages, to show that the Nudibranchiate M n i (V\s 104 1 ) differ from other Gasteropoda in the deficiency of a proper venous ivioiiusca ^r g. / ne o- at i T ed, on the one hand, by the discovery that in the Gaste* ropoda n tne al tL sy^tenfic venous circulation is 'lacunar ; whilst on the other hand, IZ NnrlUnVhiata have been proved to possess, like other Gasteropoda, proper branchio- t e dfa^X%ee a the AnaLny of Eolis, by Messrs Alder ^and Hancock, in their . u M 1 nHiV>ra.nr,hiate Mollusca " Dublished by the liay Society.) Monograph of the Nudibranchiate Mollusca," published by the Ray Society.) f Hancock and Embleton, loc. cit. Mil j ! 254 OF THE CIRCULATION OF NUTRITIVE FLUID. * opens directly into the visceral cavity, which thus, as in the inferior Mollusca, is made to take part in the venous circulation. By the union of the systemic veins on either side, the blood is conveyed, not imme- diately to the gills, as in the other Mollusca, but to two superadded impelling cavities, or branchial hearts (s), one of which is situated in con- Fig. 125. nection with each row of sills. These bran- <,■&» A' chial ventricles are less powerful than that which propels the blood through the system • but they are still sufficiently mus- cular and contractile to accelerate the circu- lation through the re- spiratory organs, and thus to the prepare blood for the susten- tation of the muscular exertions which are required for the supe- rior locomotive powers of these animals, as well as for the general activity of the func- tions of their highly- organised bodies. The blood that has been thus impelled through the branchial arteries (s'), is returned in an aerated condition by the branchial veins (£), which unite into branchio - car- diac canal on each side. These canals, before a single entering the ventricle, present dilatations, regarded as Anatomy of Octopus, the animal being laid open on the ventral side, and the inferior wall of the abdominal cavity, with the liver, having been removed : — a, a, base of the tentacula, with the suckers a' a' on their inner surface; b, head; c, eye; d, d, mantle turned back: / \ i • i i i c, funnel ; /, fleshy mass surrounding the mouth ; g, salivary glands of the \ U ) U )-> WlllOU nave been first pair ; h salivary glands of the second pair, with their suspensory rmrmllv ligaments h', and their excretory canal h" ; i, oesophagus ; /, crop ; ^ . 7c, stomach; Z, spirally convoluted csecal appendage (rudimentary pan- mere sinuses I but as creas?); ; w, commencement of intestinal tube, with biliary canal on each + l™ r 1**™ i,' ^ side; m', intestinal convolution; m", anal extremity, turned down- XjJie y nave Deen Ob- wards and to one side; w, ovarium ; n' 9 n', oviducts, of which the one is Served to be distinctlv in its natural position, and the other turned downwards ; o, o, branchiae - ^ p, heart; q 9 ascending aorta; r, venous trunk passing towards the pulmonary heart; r', its glandiform appendage; *, pulmonary heart- s', branchial artery; t, branchial vein; u ,u, bulbous dilatations of bran- chio-cardiac veins. Mollusca. — Here contractile, they must be considered as in some degree represent- ing the double auricles the complicated form of the vascular system in warm blooded animals CIRCULATION IN CEPHALOPODA AND FISHES 255 possessed of a complete double circulation j the trunks which convey the blood to the respiratory organs being furnished, like that which distributes it to the system at large, with an impelling cavity, by which a constant and regular current is maintained. In the Tetrabranchiate order, on the Nautilus sents nearly the same arrangement as in the Gasteropoda ; for the veins that return the blood from the system enter a common sinus, which has not however, a distinctly muscular character, and does not seem to possess ' ., ' -i a xi- > n „A +1^ Avt,y. 'ktvj-nr.Tiinl t.rrmks which coni.raci.iie ijuwclo, <*^ "^^ »». r - ~~~~- -_ — j v„„v distribute the blood to the two pairs of gills, whence it is conveyed back to the heart or systemic ventricle by branchio-cardiac canals.— X rom yarious parts of the systemic venous trunks, both in the Tetrabranchiate and Dibranchiate orders, a curious series of follicles or little , sacs ; is seen to proceed^ forming spongy ^^somet^sd: °™^™ aspect and the distribution of arterial branches over their surface, joined wlh the peculiar character of the fluid found in them, haye caused them to be regarded as secreting organs, destined to purify the emulating fluid ■ and it has been thought probable that they are really Kidneys. 940 The complete restriction of the circulating current to the proper vascular system, to which we have seen that the higher Cephalopoda pre- Fig. 126. ***** Anatomy of the Circulating W^^tt^ t ft£^+£1& a", bulbous dilatation of the branchial trunk ?,' *> o ^J; n ° 0r ' . %m venous trunks from cephalic veins ; b" b", great venous tnmb » ^^. 1 ^wto amfair-b adder ; e, branchial the digestive organs, liver, tadneys &™«*™° Abranchial veins, whose union forms the artery, giving off a brands ta .each ' ^gyjb^ ^g^^ £ eart beill , supplied by aortic trunk that supplies the body m ™*X^tbe branchial veins ; e, visceral branch of the arteries, d> d>, which originate ™fif ^y m t h >b™uetaaiym ^ . 2 .tamaohj aorta; *', ^^^^^^^^^^1 °, H^l ?> »™> OT ^ 3, pancreatic cseca; 4, small intestine , 0,10^ .^i™™;™ bladder. ^iXirSSe i^^aSXS^iSKfSSU it; the references as above. typical distinctions of the Ver sent a near approximation, is uue ui «™ y r ™.pvaik tebrated series, throughout which it universally prevails. For in no 1: ; I ■. 256 OF THE CIRCULATION OF NUTRITIVE FLUID. instance does the blood, in any part of its course, escape into the general cavity of the body, or diffuse itself interstitially among the tissues ; the introduction of fresh nutrient materials into the circulating current, is no longer accomplished by transudation through the walls of the alimentary canal into the surrounding space, but is effected by absorbent vessels dis- tributed upon its lining membrane (§ 1 82) ; and a special system of vessels is also provided for the re-absorption of any superfluous nutritive materials, that may have remained unappropriated by the tissues into which they have transuded from the capillary network (§ 183). — In other respects, we pass from the higher Cephalopoda to Fishes without any marked alteration in the type of the Circulating apparatus; save that which is involved in the higher provision here made for the aeration of the blood. The single ventricle of the heart (Fig. 126, a!\ the cavity of which pos- sesses firm muscular parietes, propels the blood at once to the gills, through an arterial trunk (c), which presents a bulbous enlargement (a") at its origin; this bulbus arteriosus, as it is termed, will be hereafter shown to exist at an early period of development in the higher Verte- § trunks. This trunk sub- divides into four or five branches on each side (a, g\ c', c', c'), which run along the branchial arches, sending ramifications to every filament. After being thus aerated, the blood is collected by the branchial veins, d, into the great systemic artery or aorta, which then distributes it to the dif- ferent organs of the body ; and thence it is returned to the auricle by the systemic veins (&', 6", &'"), which, before entering it, unite in a large dila- tation, the sinus venosus (6). — The circle just described appears simple in character ; but it possesses one peculiarity which is worth notice, as fore- shadowing more important modifications in higher classes. Two or three small arteries are usually seen passing-off from the branchial arches, so as to convey the pure aerated blood directly to the head, instead of trans- mitting it to the general systemic trunk. It will be hereafter shown that a similar provision exists in the Crocodile, and has a very important pur- pose in its economy ; and that the same condition is manifested up to the termination of the embryo state of the higher Vertebrata, including the Human species. — Of the blood which is being returned by the veins from the systemic capillaries, a part is diverted into another channel before reaching the heart ; for the veins of the digestive and generative organs, together with a part of those proceeding from the posterior part of the body and tail, reunite into trunks which convey the blood to the liver and kidneys; and it is minutely distributed through these organs, in order that it may undergo purification by the elimination of their respec- tive secretions. After this process has taken place, the blood is conveyed to the heart by the hepatic and renal veins, which enter the vena cava, or proceed direct to the sinus venosus. Thus the portal circulation, as it is termed, holds precisely the same relation to the general circulation in Fishes, as did the respiratory circulation in the Crustacea and the inferior Mollusca ; being interposed, for the purification of the blood which has circulated through the system, between a part of its capillaries and the heart. 241. The foregoing is the general plan upon which the Circulating apparatus of Fishes is constructed ; but there are some remarkable de- • I CIRCULATION IN FISHES 257 partures from it, which requires special no ice. The most £^££ these is that which is presented m the Amphioxus the condition oi whose sanguiferous system almost carries us back to the type of Eunice (§ ^ , for the impelling power is not here concentrated m a single organ, but is lltrloutei. among 1 a number of separate pulsatile dilatations developed upon the vascular trunks. Thus we have not only a P™P a ^^ heart (Fig. 127, ov), which corresponds in its position to the heart ot tne Fig. 127. m3 i °LL££JJL± 9\ - - - - r) 71- > ' c s - — .-— — ~ , -£■ .-''V ^ - ? -" „ ,7 ~:r~"^ — - — -'- — ._ V u ^ ^ W P h n higher Fishes; but there is also a minute contractile bulb at the ^ongm of each branchial artery, so that there are from tweniy-fi^ to ^ fifty of these bulbs on either side. The arterial arches, also, at the anteiioi ex tremity of the body, appear to be contracti e. The "^*?Wtfr is furnished with its own pulsatile dilatations; for a venous heart is developed upou the vena cava or great dorsal vein, and ™>^ W ™ , trunk of the vena port*, which runs along the ventral side of the ; ntes- tine.-Some traces of a similar arrangement may be seen m other Fishes especially in those of the Cartilaginous group Thus m Myxine theie is a portal heart, which contracts regularly, and assists m maintaining the portal circulation j in C/mncera, which has no bulbus arteriosus each oi the pair of large arteries given off from the aorta to the P^alftns,haB a contractile bulb at its origin ; and a pulsating ddatati on isfound at t he extremity of the tail of the Eel, where it receives the , ^ *om *he delicate veins of the end of the caudal fin, and propels it mto the caudal vei n.— There is another set of modifications, however, in the Circulating apparatus of Fishes, which conducts us towards the Reptilian type. For there are numerous instances in which the filaments on one or more of tne branchial arches remain undeveloped; so that the artery of that arch, Wead of subdividing into capillaries, carries-on the blood at once into Sfala; td thus the frdS transmitted ^\^^J\^ svstem has only in part undergone aeration, the head, howevei, being 3^ supplieYwith "pure arterial blood from the branc^al vem^ Con- currently ^ this change, we find some provision for a l w °^™ ™^\ ration /either in the advanced development of the an-bladde , into a rudimentary lung, to which air gains access through a tracheal canal as in the Lejdostem and Pohjpterus, the two existing representatives of the I I-. M ' *'• . I I- *■ *v f -■ *J 258 OF THE CIRCULATION OF NUTRITIVE FLUID. v great Sauroid family so abundant in the earlier periods of the Earth's history ; or in the development of peculiar organs, analogous to these in function, but not homologous in structure, being saccular prolongations of the lining membrane of one of the gill-chambers, such as are found in the Cuchia, an eel-like fish of the Ganges, and in some others of the same § The blood is sent to these pulmonary organs by prolonga- tions of the branchial arteries, and is returned from them in an aerated state into the aorta. In some of the Fishes whose Reptilian affinities are the greatest, there is a slight indication of that subdivision of the bulbus arteriosus into two distinct trunks, which takes place during the develop- ment of higher animals (§ 258). 242. Quittin water, and passing-on to the air-breathing Vertebrata, we find that very important modifications of the Circulating system are necessary, to adapt these animals to the conditions of atmospheric respiration. It is evident that the blood will be aerated much more rapidly when exposed to the air itself, than when merely submitted to the small quantity which is diifused through the watery element. If, therefore, the whole amount of the cir- culating fluid be thus exposed, the changes which it undergoes will be performed with such increased energy, that, if the other vital processes be made to conform to them, a ' warm-blooded ' animal is produced at once. But as Reptiles are intended to lead a life of comparative inertness, and to exist in circumstances which would be fatal to animals of higher organiza- tion, the respiratory process is reduced in amount, by the peculiar arrange- ment of the sanguiferous system now to be described. The ventricle of the heart is either single (which is the case in Batrachia) or it is divided by an imperfect septum (as in most of the higher orders), so that the blood which is poured into it from its two sides can mingle more or less freely in its cavity. From this ventricle (Fig. 128, a) a single truncus arteriosus is given off, which distributes the blood, by a series of arches very /) partly to off from the first and second branchial arches, and through the aorta (g, g*) which is formed by the union of the second pair with branches from the third, — and partly to the lungs through the pulmonary arteries (A, h'). In some of the higher Reptiles, the pulmonary and systemic trunks are kept distinct at their origin, by the division of the ' truncus arteriosus ' the former arising from the right, and the latter from the left side, of the partially-divided ventricular cavity ; still the general appearance of branchial arches is preserved; and a part of the blood expelled by each contraction of the ventricle, is sent to supply the requirements of the nutritive system, and a part is separated for aeration. The pure arte- rialised blood which returns from the lung by the pulmonary veins (s) is conveyed to the left auricle (c) ; whilst the venous blood which is trans- mitted by. the systemic veins (q, r) enters the right (b) ; hence these two auricles are not repetitions of one another, but have distinct functions. Both empty themselves into the ventricle, in which the blood derived from these different sources is mixed, and from which one part is again sent to the body, and another transmitted to the lungs (i). The portal system of Reptiles corresponds with that of Fishes, in the circumstance that the kidneys are supplied by it with venous blood, as well as the CIRCULATION IN REPTILES. 259 * liver ; and also in deriving part of its supply from the veins of the tail, posterior extremities, and genital organs, instead of (as m higher animals; from the veins of the diges- tive organs alone. The de- gree in which the renal, hepatic, and portal circula- tions are united, however, and in which they are sup- plied from the systemic veins, differs considerably in the several orders; the closest Fig. 128. v x b \ \ approximation b to Fishes emg presented (as might be anticipated) in the order Batrachia 243. Although the fore- going may be regarded as the general type of the Cir- culating apparatus in the class of Reptiles, yet there are some very curious mo- difications of it, some ot which connect it very closely with that of Fishes, and others with that of Birds and Mammals. The former are shown in the several genera of Perennibranchiate 299), which present us with a very com- plete series of transitional \ r r -• v. \ e ■< -" d X / **■• / U. 4* » ' t i *' n ' y- "V . ..'' c^P> r 9 k Q h /if H m R m .,'.: CS/J I > • "ft MV ' *>, a s f h 1 k..-\ V \ 1 V. A* N J *I ^ e Wi Tianli I i \ / f&l V V\ * » _* /7/\ N ft r ■ \i: m - j >' Amphibia ( § E Mm ■ fc W forms connecting the two 9 classes ; and also in the pro- gress of the metamorphosis of the Batrachia, which in their tadpold or larva con- dition must be regarded as Fishes essential ^^ — -t.y rar" \L « a i i> o Z ^ v / V / / O t A>^/#^V s \ I v / m every particular of their organiza- tion. Their circulation is for a time performed exactly upon the same plan as in that class ; the blood being transmitted from the simple bilocular heart to the bran- chial arches, then being pro- pelled through the branchial filaments, and after aeration being circulated through the system. The mode in which the transition is effected from^w^ of the vascular organs, to that which they present m the perfect Reptile state S A Circulating Apparatus of Lizard :—a, single ventricle ; b right auricle; e, left auricle; d d', right and left aortic arches ; e, carotid artery ; /, brachial artery; g,g', ventral aorta; h,K', pulmonary arteries; i, lung; k 9 stomach; I, intestine; m, liver; n, kidney: o, vena portse; p, gastric vein; q t vena cava ascendens; r, vena cava descendens; s, pulmonary veins. 1 V 260 OF THE CIRCULATION OF NUTRITIVE FLUID. * of the animal, described in the preceding paragraph, will be rendered intel- ligible by the accompanying figures. In Fig. 1 2 9 is seen the arrangement Fig. 129. ■>xS^ X i li Respiratory Circulation of the Tadpole in its first state:— a, arterial bulb, giving off three pairs of branchial arteries, b, to supply the gills, b\ & 2 , 6 3 , from which return the branchial veins, c c; by the union of the second and third of these are formed the two arches, d d, which coalesce to form the aortic trunk e; from the first pair are given off the cephalic arteries,/,/; and another trunk, g, is derived on each side from the aortic arch; h, h, pulmo- nary arteries as yet rudimentary; between the branchial arteries and their respective veins, at the base of the gills, are minute inosculating twigs, the position of which is better seen in the succeeding figures. of the parts before the metamorphosis has commenced. Three branchial trunks (b) pass off on each side of the heart, terminating in a minute capillary network which is contained in the branchial arches (b\ b 2 , b 3 ), and by which the blood is aerated during the aquatic existence of the animal ; from this network the returning vessels take their origin, which unite into trunks (c, c), one for each gill ; and of these the first gives off the mam arteries (/, /) to the head, while the second and third unite into the two trunks (d, d) which coalesce to form 4he great systemic artery (e), as m Wishes But, besides these vessels, there are some small undeveloped branches, which establish a communication between each branchial artery and the returning trunk that corresponds with it. There is also a fourth small trunk given off from the heart, which unites with another small branch from the aorta, to form the pulmonary arteries (A, A) which are distributed upon (as yet) rudimentary lungs. After the meta- morphosis has begun, however, by which the Fig. 130. hi P& the animal from a Fish has to be converted into a Reptile, the branches (Fig. 130, i, 2, 3) that connect the arteries of the gills with their . m , . . returning trunks are The same in a more advanced state; the communicating twigs, muc ], increaSPrl in nw 1, 2, 3, as well as the pulmonary arteries, h, h, being now greatly en mucn increased m Size, larged m proportion to the branchial vessels. m b so that a large part of . 11A , x1 , ,. >.i , , . tlie ^lood flows conti- nuously through them without beino- s^nt +^ +1^ ™-n« «+ .n „j n . CIRCULATION IN REPTILES 261 branchial vessels (b\ &»,») are themselves relatively dimi ^;^*J* the same time, the pulmonary trunks (A, h), which were before the smallest, become the largest, so that ~- increased proportion of blood is sent to the lungs. By a con- tinuance of these changes, the bran- chial vessels gradually become ob- literated, and the communicating branches, which were at first like secondary or irregular channels, now form part of the continuous line of the circulation (Fig. 131); the upper one sending off the cephalic vessels, the second and third uniting to supply the trunk, and the fourth passing as before to the lungs. _ Turning from these to the Perenm- | branchiata, we find in the Lepido- \ siren a plan of circulation but little \ elevated above that which has been just described as existing in certain Fishes that present approximations to the Reptilian class. mi " cular cavity is single, and gives off but one primary trunk, which is furnished with a ' bulbus arteriosus.' Fig. 131. The same in the perfect Frog; the vessels of the ml j. • branchiae b\ V, 6 3 , being now atrophied, the com- The Veiltri- mullica ting twigs l, 2, 3, now become the principal channels for the direct passage of the blood from the branchial arteries to the cephalic vessels/, g, tne aortic arches d, d, and the pulmonary arteries h, h. turnished with a ' bulbus arteriosus. This trunk subdivides into six pairs of branchial arteries; of which the 1st, 4th, 5th, and 6th are distributed to the branchial fringes whilst the 2nd and 3rd are not thus distributed, their arches having ™^J^f* to them, but reunite to form the aortic trunk firs t giving off, ho w ,er the pulmonary arteries. Thus the blood which finds its way into the aorta consists partly of that which has been aerated m the branchy and partly of that which has passed to it direct from the heart. But thaV which m transmitted from the heart is itself of mixed quality, as in Septal es ^f «£ pulmonary vein passes through the systemic auricle, and .^ohar^es itedf directly into the ventricle, where its aerated blood is mingled with that rZnldf y the slstemic vein,-Xn the Proteus the -%&£«£*» vascular system permanently resembles that which has been ^piesented as intermediate between the larva and the perfect condition of the *rog. This animal is provided with lungs slightly developed, as well as with permanent gills f and the blood which is expelled from the ventricle is parSy transmitted through the gills, partly finds its way directly into the aorta by means of the communicating branches, while a small quantity is transmitted to the lungs; the latter is returned perfectly arteriahsed to thTauricle here developed upon the pulmonary vein, and is afterwards mL Tin the Ventricle with the venous blood transmitted from the sys- temic auricle. aiVTTmanv of the higher Reptiles, on the other hand, we find the cavity of the ventriele more or hi perfectly divided into wo, and the : J Ui l , .. _ ™™^i^^.t ^naroi,P( from the systemic. separated from tlie systemic, pulmonary circulation i^xv ^~ r ^y ~^* mm+nnia Thus, in the Lacerta ocellata (spotted lizard), whose venture pulmonary circulation more ipletely partly ii 5. i 262 OF THE CIRCULATION OF NUTRITIVE FLUID. divided, the right side of it, into which the systemic auricle discharges itself, gives off the pulmonary trunks, so that a large proportion of the venous blood brought back from the system is transmitted to the lungs for ZnlZ\ 7 •? 1 % be i ng retUrned t0 the Penary auricle, is discharged into the left side of the ventricle, from which the systemic arteries pro- ceed As long as there is any direct communication, however, between f!l T t the W ' lt 1S ° bvi0US tbat a P^ of the blood returned tSJTi^ Vei ^ may be S6nt immec ^tely into the aortic trunks, without being previously arterialised ; whilst in proportion to the decree tnJI^ ^ "'A " °° m P lete > wil1 *e the approach of the annual towards the condition of the warm-blooded Vertebrate. The distribution of the vessels has a considerable effect upon the character of the fluid with which individual organs are supplied ; for in Reptiles which mani- fest tli is separation to a considerable extent, a part of the blood trans- mitted to the system has still a venous character, whilst that which is fiirnished to the brain and upper part of the body is purely arterial, ihis difference arises from the fact, that of the two arches which unite to form the aortic trunk, one is connected with the right and the other with the left side of the ventricle ; the latter receives chiefly the arterial blood from the left or pulmonary auricle, and this gives off branches which convey it without admixture to the head ; while the main trunk passes on to unite with the second aortic arch, which arises from the right side of the heart and which is consequently supplied chiefly with venous blood, that has been brought back from the system into the rteht auricle This second arch, before its union with the first, however, gives off a large branch, which is distributed to the intestines and other viscera, and which therefore, contains venous blood with little admixture of arterial ; and mixenril° rtl ? tnmk ' ^T med hj the union of tbe two arches > conveys berT Tt Tb + '7i T b °° d t0 tlie remai *der of the trunk and mem- teaiest economv J + ° ^t W hj tbeSe sim P le contrivances the greyest economy of material is obtained, whilst each organ is supplied with blood sumciently oxygenated to maintain its functioBS.-ThXco ttCTlZ Tf ; l^tf ° f ^ VaSCukr **» still more aC If °^ arm : bl00d + ed Vertebrate; the ventricular septum being com- plete and the circulation, as far as the heart is concerned, being truly double, bull, however, whilst the principal aortic trunk arises from the left ventricle, which contains nothing but arterialised blocd, a second arch arises from the right (or venous side) along with the pulmonary artery, of which it might almost be considered a branch ; and this after giving off its intestinal branches, enters the first trunk, which has already furnished the cerebral arteries with pure arteria] blood, and transmits the mixed fluid to the rest of the system. There is another communi- cation between the trunks arising from the two sides of the heart by means of an aperture which passes through their adjoining walls test after their origin; so that although the blood in the heart is entirelv venous on one side and arterial on the other, it undergoes admixture in the vessels according to the character of the functions to which it is to minister. We shall presently see a remarkable analogy to this distribu- tion of the vascular system, exhibited in the foetal condition of Birds and Mammals (§ 258). 245. In the highest form of the Circulating system, that possessed by c CIRCULATION IN BIRDS AND MAMMALS Mamr 263 Fig. 132. /> " : F v^*< the ' warm-Diooaeu v tuLeux^a,, — «~ — . -~ ~- : , ra^d double circulation of the blood; each portion of it, which has passed through the capillaries of the system, being aerated m the lungs, before being again distributed to the body. This is effected by a form of the vascular apparatus, of which a sketch was pre- sented in the Cephalopoda (§ 239), and to which a near approach is exhibited by the higher Reptiles. The heart consists of four cavities, two auricles and two ventricles j those of the right or venous side having no direct communication with those of the left or arterial side ; and the vessels proceeding from them being entirely distinct, and having no connection whatever, except at their capillary terminations. The blood transmitted by the great veins of the system to the right auricle or re- ceiving cavity, passes into the ventricle or propel- lino- cavity, and is transmitted by it through the pulmonary arteries to the lungs of the two sides. After being there arterialised by exposure to the atmosphere, it is brought back to the left auricle j and having been poured by it into the correspond- ing ventricle, is transmitted through the great systemic artery or aorta to the most distant parts of the body (Fig. 132). The heart is there- fore completely duplex in structure, and, so far as its functions are concerned, might be regarded of two distinct portions; consisting for Diagram of the Circulating ap- paratus in Mammals and Birds: —a, the heart, containing four cavities; b, vena cava, delivering venous blood into c, the right auricle; d, the right ventricle propelling venous blood through e, the pulmonary artery, to/, the capillaries of the lungs ; g, the left auricle, receiving the aerated blood from the pulmonary vein, and delivering it to the left ventricle, h, which propels it, through the aorta, i, to the sys- temic capillaries, j, whence it is collected by the veins, and car- ried back to the heart through the vena cava, b. economy of material, however, these are united, the partition between the ventricles serving as the wall to each. In the Dugong (one of the aquatic Pachydermata), however, the heart is bifid at its apex, and thus presents a partial division into two separate organs, not only functionally but structurally.— The portal circulation is limited in Mammalia to the liver, the kidneys being sup- plied with arterial blood only. In Birds, however, we find a trace of that arrangement of this peculiar offset from the general circulation, which has been pointed-out as existing in Reptiles and Fishes ; for the great portal trunk receives its blood, not only from the veins of the digestive apparatus, as in Mammalia, but also by branches fromtliose of the pelvis and posterior extremities ; and it still communicates with the renal circu- lation, although this connecting branch seems rather destined to convey blood 'from than towards the kidney. 246 Various peculiarities in the distribution of the sanguiferous system, which are presented by different orders of Birds and Mammals, would be worthy of notice if our limits permitted Of these, one of the most remarkable is the modification both of the venous and arterial trunks, existing in the Getacea and other diving animals, which are occa- sionally prevented from respiring for some time, and m which therefore, the arterialisation of the blood is checked. Various arteries of the trunk are here found to assume a ramified and convoluted form, so that a large i I ■^HHH * 264 OF THE CIRCULATION OF NUTRITIVE FLUID. quantity of blood may be retained in the reservoirs formed by these plexuses ; whilst the venous trunks exhibit similar dilatations, capable of being distended with the blood which has been transmitted through the system so as to prevent the heart from being loaded with the impui e fluid, whilst the lungs have not the power of arterialising it. In some diving animals, this object is effected, not so much by a number of venous* plexuses as by a single great dilatation of the vena cava before it enters the heart, resemhlmg the 'sinus venosus' of Fishes—In other instances, the force with which the blood is sent to particular organs seems to be purposely diminished, by the division of the trunk that conveys it, into a number of smaller vessels, which, after a tortuous course, unite again and are distributed m the usual manner. A structure of this kind is found in the arteries of the long-necked grazing animals, to which the blood would be transmitted with too great an impetus, owing to the additional in- fluence of gravitation, were it not retarded by such a contrivance A similar distribution of the arteries is found in the trunks supplying the limbs of the Sloths, and of other animals which resemble them in tardi- ness of movement. In other cases, the arterial canals are specially pro- tected from compression by surrounding organs, in order that there may be no obstruction to the passage of blood through them, and that they may be guarded from injury; thus, in the fore-leg of the Lion, where all possible force and energy is to be attained, the main artery is made to pass through a perforation in the bone, as if to secure it from the pres- sure oi the rigid muscles, which, when in a state of contraction, mi S ° that the S *P™ s ^ngth and and^dLatio^ b ^ SCem to be not together the result of habit and education as some have supposed; in Birds, however, where any inequality m the powers of the two wings would have prevented the necessary regularity in the «««— ~» ^•■. ■■■ ■ r^""""^ *^« actions of flight, the aorta gives off its further branches to the two sides with perfect equality netted (^e^ diStributi ° n ° f the arterial 4*m will be hereafter 247. Having now traced the Sanguiferous system to its highest form it is proper to inquire how far this differs functionally from that simple condition which it presents in the lowest tribes in which it has any dis tmct existence. There can be no doubt that, in the higher animals pos- sessed of a distinct muscular heart, this is the chief agent in keeping-™ by its successive contractions and dilatations, the motion of the blood through the vessels. But a careful survey of all the phenomena of the circulation would seem to lead to the conclusion, that the impulse of the heart is not the only means by which the motion of the blood is sustained ; but that an additional impulse is given by the contraction of the muscular walls of the arteries, upon the jets of blood successively impelled into them by the heart; and that the changes which this fluid undergoes in the capillaries have some share in its production, and have at any rate a very considerable modifying effect upon the quantity transmitted through the individual organs. We have seen that in Vegetables the laticiferous circulation is entirely capillary; that in the Holothuria there is no central contractile organ which seems powerful enough to impel the blood throucdi i FORCES MAINTAINING THE CIRCULATION, 265 all the minute ramifications of its vascular system; whilst ewam^e Wl r ArtLlata, and in all Mollusca save the ^ephalopods, so large a part of The svstemic circulation is < lacunar,' that it seems impossible to imagine Jhat th Jttion of the heart can urge the blood through the branchial that tiie action ^^ f ^ ^^ , ^^ thereforej wou ld vessels which, succeed. are anions the facts which eem to be in operation; and the following are ^ *£*^™£ annear to support the conclusion, that, even m the highest animgJb, tnese S "eneJ JLces are not obliterated, but are merely superseded by the rerVof the special organ, which is developed as the centre of the whole Sltion and" which fs endowed with an amount o ? P™£^ govern and harmonise the numerous actions gomg-on m differ ent parts ° f 248 Sy in e many warm-blooded Vertebrata, and still more in the cold- blo^edWi^ .^jrt**ST S yS t^TTLlT^t ZT d ::,t^^^:i^:^ checked by certain applica- tions to the parts themselves. After most kinds of slow natural death, he atrial Lnks and left side of the heart ^^^^°^ even completely empty, and the venous cavities to be full of ^ Wood iiiis StZs P been y ascled to the contraction of the arterial tubes ^after the heart has ceased to beat; but it seems impossi bh , that ^nmLhed n due to that cause, since their calibre is not ffound to ^jju™£ ^ a proportional degree; if Hjj-b. ^^^^SS^^t subsequently to general or somatic death, affords an exceUent F^ ^ lingering vitality ; and it is scarcely possible that these car, ^"^Xtf witW'some degree of capillary circulation. There are ™^»L ° sudden death, however, in which the vitality of ^.^^ZTIT^ to be simultaneously destroyed, anch to , blood remains m ^th ^^ M ^* was at the moment of decease—Further, a careful «^^~ ™ circulation in the living animal discloses many ™^™™^J^ of the capillary currents, which it is impossible to attribute , tc an influe ^ derived from the heart or from the vessels that ^^Tetwork of a variations may present themse Ives^er m ^he cg*% -^ part, or m a portion of it , the « ^another, though both rapidity m one spot, and with m create ± ennj ' extendg to ft are mirmlied by the same trunk. ±ne cnau^e wu ^ ™mmete rtJsal of the direction of the moveme.it, m certain of the Svel oi c 0mmu nieating branehes ; this movement takrng place of S^Tom the longer toward^ ^^"wio^^t T n ^ ?*££g£*££££ X kind are most freonent SrT taffii ™fe*led or partially intend ; and it • B .*. teen otaerred by D, «• *£**» ^*KL53ffitai hare died of yollow fever, the external vans ™qnenuy » that, when they are opened, wilhhi a/e» ».;»«<» after tae cessation oi the njaresaoe , Ncvr 0rleaI , s Mod. and fall stream, as in ordinary blood-letting Surg. Journ.," Jan., 1849.) the blood flows in .. If! I u i I 266 i I J ■ OF THE CIRCULATION OF NUTRITIVE FLUID. would thus appear that the local influences by which they are produced, are overcome by the propelling power of the central organ, when this is acting with its full vigour. When the whole current has nearly stagnated and a fresh impulse from the heart renews it, the movement is seldom uniform through the entire plexus supplied by one trunk : but is much greater in some of the tubes than in others,— the variation being in no degree connected with their size, and being very different in its amount at short intervals. 249. Amongst the most remarkable proofs of the influence of forces generated m the Capillary circulation, on the general distribution of the blood, is one derived from the observation of organs which undergo changes in activity that are quite independent of alterations in the heart's action. Thus, when the uterus begins to develope itself during pregnancy, the unusual activity of its nutritive operations induces an increased de- mand for blood in its capillary circulation, which is supplied by an in- crease in the diameter of the trunks that transmit fluid to the omxn • and this is entirely independent of any increased energy in the heart's action, which would have affected the whole system alike. The same may be said of the occasional development of the mammae for the secre- tion of milk ; of the rush of blood through these organs during the act of suckling; and of similar changes in other parts, of which the° activity is not constant or uniform. In certain diseased states, also, of particular portions of the system, which do not occasion any appreciable alteration m the heart's action, the quantity of blood sent to the part is much in- creased, and the pulsation of the arterial trunk leading to it is evidently stronger than that of the corresponding vessels on the outside of the body These phenomena, and many others which might be mentioned, are evi- dently analogous to one formerly stated as- having been ascertained bv experiments on Plants (§ 201) ; and, when taken in connection, they seem to indicate without much doubt, that the quantity of blood sent to indi- vidual organs, and the force with which it is transmitted through them are augmented with any increase of energy in the vital processes taking place in them, the vis a tergo derived from the impulsive power of the heart remaining the same.— Additional evidence of the influence of the forces generated in the capillaries, on the general circulation, is derived from cases in which the normal changes to which the capillary circulation ministers are suspended, and in which it then appears that the heart's impulse is not alone sufficient to maintain the current of blood. One of the most conclusive of these proofs is drawn from the phenomena of Asphyxia, or suffocation; since it now seems distinctly ascertained, that the check given to the circulation, and thence to all the other functions, arises from the stagnation of the blood in the capillaries of the lungs', consequent upon the cessation of th e reaction between that fluid and the air.* So again, cases of spontaneous gangrene of the lower extremities are by no means of unfrequent occurrence, in which the local stagnation of the circulation has been clearly dependent upon the cessation of the * For a fuller discussion of this part of the subject than the limits of this treatise per- mit^ see the Author's "Human Physiology" (§ 575), and his Article on Asphyxia in the Library of Practical Medicine." See also the very conclusive experiments of his late valued friend Dr. John Eeid, in the "Edinb. Med. and Surg. Journal" for April 1841 • and Dr. Reid's " Physiological, Pathological, and Anatomical Researches," chap, ii Jfc FORCES MAINTAINING THE CIRCULATION 267 nutrient actions to which it was subservient ; it being found, "by examin- ation of the limb after its removal, that both the larger tubes and the capillaries were pervious throughout, so that no mechanical impediment existed, to prevent the propulsive power of the heart from transmitting the blood through them. The influence of the prolonged application ot cold to a part, may be referred-to in support of the same general proposi- tion • for although the calibre of the vessels is diminished by this agent, vet their contraction is not sufficient to account for that complete cessa- tion of the flow of blood through them, which precedes the entire loss ot their vitality.— A periodical retardation or suspension of the circulation in particular portions of the body, unaccompanied by any other ostensible change, and not dependent upon any failure of the heart s power is by no means an uncommon phenomenon. It frequently presents itself, foi example, in one of the fingers; and a curious <^ " ^£,^1 Graves * in which the whole of one leg was thus affected with lemarf: airperiodiclty, for about twelve hours out of the twenty-four ; whilst m the i tTvals the circulation in the limb was unusually active the action of the heart being quite natural throughout, and the circulation m the rest of the body not being in the least affected. 250 In the development of the embryo of the higher Yertebrated animals moreover, there is a period at which a distinct movement of red blTd L is seen before any pulsating vessel can be detected to possess an Muence oTe'r it (§ 255). Further! instances not very unfrequently occur of fetuses having attained nearly their full devdj^^h have been unpossessed of a heart, and in which the circulation has been, as it - were entirely capillary; and although m most, if not all of these cases, ^ e monster Ls Leu "accompanied by a perfect child, «o« may have been suspected to have influenced its ^^T^H'as 7 been oJ of those most recently examined, the occurrence of this has been disproved. From a careful examination of the vas ^i system, it ap peared impossible that the heart of the twin ^ <^ ^erefore the movement of blood in the imperfect one ; and th is murt, ^or e , have been maintained by forces arising out of the nutntive changes ° C 2Ti rin iir ttrcitnSLes indicate that the movement of blood Jo^e C^es i, very ^XfTl^i^^^ 2SSTS* SSa^H^' And froni" other facts 1 ap^ that the conditions necessary for ^e^g**^ Hood through these vessels, are nothing else than the active performance ot the nutritive and other operations, to which its movement is subservient. The principle already noticed (§ 205) as having been developed by Frof Dr^)er, P seems fully adequate to explain these phenomena. It will * « Lectures on Clinical Medicine," second ^^Ittml^cliei by Dr. Houston of + For the details of this mt eresting case winch ( ™^™ OTeign Medica i Review , » Dublin to the British Association in 1836 see the Bntish^nu * , ^ ^^ Vol. ii. p. 596, and the ' ' Dud in Medical Journa 1 for 18 37. A ^ S 0T1sWb infei -. Dr. Marshall Hall ("Edinh. Monthly Journal, 1843) to asp ^^ j^^,, { ^ ences; but a most satisfactory reply was made by Vv. n. 1844.— A similar Jan. 1844. See also the " Edmb. Med and Smg. Journal, ^ ^^ ^ case is recorded by Dr. Jackson, of Boston (N.E.) in tne Medical Sciences," Feb. 1838. ' ] 268 OF THE CIRCULATION OF NUTRITIVE FLUID. be convenient to take the Respiratory circulation as an example of its application ; since the changes to which this is subservient, are more simple than those which take place elsewhere. The venous blood trans- mitted to the lungs, and the oxygen in the pulmonary cells, have a mutual attraction, which is satisfied by the exchange of oxygen and carbonic acid that takes place through the walls of the capillaries ; but when the blood has become arterialised, it no longer has any such attrac- tion for the air. The venous blood, therefore, will drive the arterial before it, m the pulmonary capillaries, whilst respiration is properly going on : but if the supply of oxygen be interrupted, so that the blood is no longer aerated, no change in the affinities takes place whilst it tra- verses the capillary network ; the blood, continuing venous, still retains its need of a change and its attraction for the walls of the capillaries • and its egress into the pulmonary veins is thus resisted, rather than aided' by the force generated in the lungs. — In the Systemic circulation, the changes are of a much more complex nature, every distinct organ attract- ing to itself the peculiar substances which it requires as the materials of its own nutrition ; and the nature of the affinities thus generated will be consequently different in each case. But the same principle holds good in all instances. Thus, the blood conveyed to the liver by the portal vein, contains the materials at the expense of which the bile-secreting cells are developed ; consequently the tissue of the liver, which is prin* cipally made-up of these cells, possesses a certain degree of affinity or attraction for blood containing such materials ; and this is diminished so r* *-\ s-\ is* *-» m -J- L-* ^ ■_— 1* . T 1 r* • , • . . -m ^ drawn will Conse- vein, the blood which has already traversed the capillaries of the portal system, and which has given-up, in doing so ; the elements of bile to the solid tissues of the liver. • ! 52 ; i ^ ( l 1 are n °T- P re P ared > therefore, to understand the general prin- ciple, that the rapidity of the local circulation of a part will depend in great measure upon the activity of the functional changes taking place in that^ part— the hearts action, and the state of the general circulation, remaining the same. When, by the heightened vitality, or the unusual exercise, of any organ, the changes which the blood naturally undergoes in it are increased in amount, the affinities which draw the arterial bfood into the capillaries are stronger, and are more speedily satisfied, and the venous blood is therefore driven out with increased energy. Thus a larger quantity of blood will pass through the capillaries of the part in a given time, without any enlargement of their calibre, or even though it be somewhat diminished ; but the size of the arteries by which it is con- veyed soon undergoes an increase, adapting them to supply the increased demand. Any circumstance, then, which increases the functional energy of a part, or stimulates it to increased nutrition, will occasion an increase in the supply of blood, altogether irrespectively of any change in the heart's action. This principle has long been known, and has been ex- pressed in the concise adage " Ubi stimulus, ibi fluxus ;" which those Physiologists, who affirin that the Circulation is maintained and governed by the heart alone, cast into unmerited neglect. 253. The development of that Circulating system which has been described as peculiar to the higher classes of Vertebrated animals, is not - <" • DEVELOPMENT OF CIRCULATING APPARATUS. 269 completed until the moment of birth j and the progressive changes i which the heart and vascular apparatus ^^^^^^Z^JX Mammals § ^^^^^^oi^:^^ in the higher animals and the forms permanently exhibited by the lower. It has been seen that m the organs P of Circulation, as well as in all others, the tendency, as we rise from their lowest to their highest condition, is one of specialization. In he si^lest Animals, as in Plants, whatever ^/* *£££££ is effected in each individual part hy and for itself; whilst m the com plex and highly-developed structures that occupy the other extremity S L "cale^the evolution of a powerful organ of ™P^»> he influ- ence of which extends over the whole system, has superseded the diffused aoencv bv which the circulation was previously maintained. Tins pro- ofe s from I more general to a more special type is equally manifested m the vascular system of the embryo ; and the analogy which thus arises between the forms it presents at different epochs of its development, and tWmesented by the lower tribes of animals, is not superficial only but SKto minute particulars. The egg of the Bird affords the best opportunity for studying the early changes which it undergoes and these have been determined with great minuteness; but such a sketch of them only can here be given, as will serve to illustrate the principles alluded to The preliminary stages of the process will be described m their W T,. IraiTarry pLd of incubation, the yolk is found to be enve- loped by a 'germinal membrane,' composed of distinct cells, which is dSbfointo three layers j and a thickened portion of this is easily dis- tinguishable, at which the embryo will be subsequently evolved. The middle layer gives origin to the Circulating system, and is there- fore termed the vascular layer. The thickened portion of this, that surrounds the germ, soon becomes studded with numerous irregular points and marks of a dark yellow colour; and as incubation proceeds, these points become more appa- rent, and are gradually elongated into small lines, which are united together, first in small groups, and then into one network, so as Fw. 133. to form what is called the Vascular area (Fig. 133). A large dark spot of a similar kind is seen m 4-L „-+„«tf«n to be subsequently Vascular Area oiFowl's egg, at the beginning of the the Situation tO De SUDSequ^ry ayo f incubation ;-»,« ,yolk; M,M, venous «««,-. y^«w1 W +.Tia heart. These ^L.i^ainf? the area; c, aorta ; d, punctumsahens, occupied by the heart. Occupied Dy uie uo<*,±u. *.^^~ sinus bounding u^^*, ->— v- .<- . f f arte ~^ a f A 1 ■ 4- ~„A li^oa qw formed or incipient heart; e,«, area pellucida,/,/, arteries ot dark points and lines are lormea the v J eular area ;<,, ^f even after the ventricles have been separated , ™*™g**?^£ the fifth or beginning of the sixth day m the OhicK, flattened and the opposite sides adhere together, so as to divide it into naraeneu, cwiu. w«> ^ +1^^ one communicates with the two tubes running side by side. Ot these, one cox left and the other with the right Tentacle. The former which subse- auently becomes the ascending aorta, a', is continuous with the fourth branchial arch, 4, on the right side only; but from tins the carotid and tachial arteries arise by two principal trunks. This arch becomes Suallv larger, so as to form the freest mode of communication between the heart and the descending aorta; it subsequently becomes m fact, the the heart ana fe ^^ . g connected Wlth the right arch of the aorta. ± ^ subsequently becomes the ;=^ transmits its'blood through t^ -W ™ side a' and the fifth arch, 5, of the right (the two primary tubes twist- ing round each other) ; but the fifth arch on the left side, 5 , now ceases !l * v*r i I 274 OF THE CIRCULATION OF NUTRITIVE! FLUID. Mammals, Fhe fourth And the to convey blood From the two trunks, 4', 5, which still discharge thsir blood into the descending aorta, the pulmonary vessels, p, p, branch off as the lungs are developed ; and the prolongation, 1 o, of the former of these, which previously constituted one of the arches of the descending aorta, soon afterwards becomes impervious. The original prolongation, 9, of the latter trunk which meets the descending aorta, still remains : so that a portion of the blood sent from the right ventricle is transmitted through this communicating branch directly into the descending aorta, j ust as m the adult Crocodile. After the first inspiration, however, the ^TT-u 1 bl °° d transmitted through the pulmonary artery passes into the lungs, and does not enter the aorta until it has been returned to the heart ; and this communicating vessel, which is termed the ductus arteriosus, soon shrinks and becomes impervious; Thus the third pair of branchial arches becomes converted into the two arterice innominatce, or common trunks, from each of which, in Birds and in some the carotid and subclavian arteries of one side originate, branchial arch of the right side becomes the arch of the aorta. fifth branchial arch of the right side, with the fourth of the left, the right and left pulmonary arteries.— The general plan of the changes which occur in the vascular system of the Mammalia (Fig. 1 35), is the same as that which has been described in Birds, the differences being only m detail ; as for instance, that the aortic arch is formed, not from the right, but from the left branchial arch. 259 Up to the period of the hatching of the egg in Birds, and the separation of the foetus from the parent in the Mammalia, the circulation retains some peculiarities, characteristic of the inferior type which is per- manent m the Reptile tribes. Of the blood which is brought by the venous trunks to the right auricle, part has been purified by transmis- M™ lA ^ff ^ or g an (^e allantois in Birds, and the placenta in Mammals^ whilst a part has been vitiated by circulation through the system. The former, returning by the umbilical vein (Fig. 135%), is mixed m the ascending vena cava 0) with the blood which has circulated through the lower extremities • whilst the descending cava brings back that which has passed through the capillaries of the head and upper ex- tremities, and which, having received no admixture of arterial blood, is not fit to be again transmitted in the same condition. It will be recol- lected that a communication still exists between the two auricles the ' foramen ovale' yet remaining pervious ; and by a fold of the lining membrane of the right auricle, forming the Eustachian valve, the ascend- ing and descending currents are so directed, that the former (consisting of the most highly-arterialised blood) passes at once into the left auricle" whilst the latter flows into the right ventricle. * From the left auricle' the arterial blood is propelled into the left ventricle, and thence through the arch of the aorta to the vessels of the head and upper extremities, a comparatively small part finding its way into the descending aorta. The venous current is propelled through the pulmonary artery; but the lungs not yet being expanded, little of it is transmitted to these organs, and #■ The peculiar course taken by the Mood through the heart, which was suspected from coloured injections, Dr. Reid's -Physiol., Pathol., and Anat. Re^rc^' Chap", t^" ^ " ^ 308; ** „« j. • V' —~""~~ ~ , J — . „ ""ougn tne neart, winch v anatomical mvestigation has 1 been actually demonstrated by means of c by Dr. J. Reid. See "Edmb. Med. and Surg. Journ " Vol xliii ™ Br. Rdd's "Physiol.. Pathol., and Anat, TfcLJS', rZ± ^ „' PP ' DEVELOPMENT OF CIRCULATING SYSTEM 275 the greater part finds its way through the ductus arteriosus into the de- scending aorta, where it mixes with the remainder of the first-mentioned portion. This trunk not only supplies the viscera and lower extremities (which are thus seen to receive, as in Reptiles, blood of which only a portion has been oxygenated), but sends a large proportion of its contents to the umbilical vessels, by which it is conveyed to the oxygenating organ, and returned again to the venous trunk of the abdomen. 260. The course of development of the Yenous system exhibits not less remarkably than that of the Arterial, a gradual passage from the more general type common to all Yertebrata at an early period of their with Mammal There is at first a pair of anterior venous trunks (Fig. 135, A, B, g, g') Fig. 135 A. B A.— Diagram of the Circulation in the Human Embryo and its appendages, as seen in profile from the right side, at the commencement of the formation of the Placenta : — b . The same, as seen from the front :—a, venous sinus, receiving all the systemic veins; b, right auricle* b f left auricle; c, right ventricle ; c', left ventricle ; d, bulbus aorticus, subdividing into e V ^branchial arteries ; f, f, arterial trunks, formed by their confluence ; g, g', vena vjght afterwards disappears, the iett being alone iully developed; q, omphalo-mesenteric vein ; r, omphalomesenteric artery, distributed on the walls of the vitelline vesicle t; v } ductus venosus; y> vitelline duct; z, chorion. Wol #) T 2 . i =7-fc 276 OF THE CIRCULATION OF NUTRITIVE FLUID. in most Fishes where they are designated the ? cardinal' veins : but azy veins (major and minor), which coalesce into a common tnmk for a con- siderable part of their length. One of the anterior trunks unites with one of the posterior on either side, to form a canal which is known as the ductus Cuvien ; and the ducts of the two sides coalesce to form a shorter mam canal, which enters the auricle, at that time an undivided cavity This common canal is absorbed (so to speak) into the auricle, at an early period, m all Vertebrata above Fishes, so that the two Cuvierian ducts terminate separately in that cavity; and after the septum auricu- lorum has been formed, they enter the right auricle.— This arrangement is persistent m Birds and in the inferior Mammals, in which we find two yense cavse superiores/ entering the right auricle separately- but in the higher Mammals, as in Man, the left duct is obliterated, and the right alone remains to form the single vena cava superior, a transverse branch being formed to bring to it the blood of the left side. Man The double vena arrest ot development. As the anterior extremities are developed, the !it I 1 ™ 11 J einS f re f ? rmed , t0 . return the W °od from them ; and these The ' omphalo-mesenteric' vein arular , . . w < -- j-^,^ — . ^^.^ ^iij.jjxi«.iw-iii^ocaj.tciHJ vein W, which, is another primitive trunk common to all Vertebrata, is formed by the confluence of the veins of the yolk-bag and of the intestinal canal, and passes by itself, with the two Cuvierian ducts, into the auricle. The upper part of this remains to constitute the upper part of the ' inferior . . *. ^ ^*v. Wolffian .juuiea, a,nu originally enters the omphalo-mesenteric vein above the liver. When f, If",! 8 ™ ed ' the omphalo-mesenteric vein becomes connected with it botH by afferent and efferent trunks, the former remaining as the vena port*, and the latter as the < hepatic vein ;' and after giving off the former trunks, the omphalo-mesenteric vein is itself obliterated, so that all the Wood which t brings must pass through the liver. The inferior cava, which receives the hepatic vein, is gradually enlarged by the reception of most of the veins from the inferior part of the trunk and the lower extremities, and the vena azygos is reduced in the same proportion ; m some rare cases of abnornal formation, however the vena cava fails to be developed, and then the blood from the lower parts of the body is conveyed to the superior cava through the azygous system. * The umbilical vein, which is at first developed in connection with the allan- tois, and which consequently does not exist where that organ is not evolved, increases in size as the mesenteric artery diminishes ■ the Greater part of its blood is discharged into the vena portse, and only reaches the inferior cava after passing through the liver ; but a part of it passes- on to the vena cava through a direct channel, which constitutes the ' ductus venosus.' A similar direct communication between the portal system and the vena cava exists permanently in Fishes, and to a less degree in other oviparous Vertebrata : and it seems there intended to transmit directly to the heart whatever proportion of the blood, brought to the * For the details of the changes above described, see Rathke ' < Ueber den Ban nnd die Wwkehmgdes venen systems der Wirbelthiere, " 1838 ; and Mr. Marshall's Memoir T\i n ! ,, e T ^P ment of the Great Ant enor Veins in Man and Mammalia, ' in " Philos. transact., 1850. * DEVELOPMENT OF CIRCULATING SYSTEM. 277 vena portse, may be at the time superfluous as regards the function of the liver. Mammal no more blood than is required for distribution through the liver ; and the ductus venosus speedily shrivels into a ligament. 261. Thus we have traced, in the development of the Circulating apparatus of the higher Vertebrata, the same progressive advance from a more general to a more special condition, as that which we have wit- nessed in ascending the Animal series ; and when considered analogically rather than homologically (§ 8), the correspondence is extremely close. For in the state of the circulating system in the early embryo, when the heart is as yet but a pulsating enlargement of one of the principal trunks, and the walls of the vessels are far from being complete, we have the representation of its condition in the higher Radiata, and in the lower Articulata and Mollusca. In the subsequent division of the cardiac cavity into an auricle and a ventricle, an advance is made corresponding to that which we encounter in passing from the Tunicata to the higher Mollusca. And when the branchial arches are formed, which enclose the pharynx and meet in the aorta, the type of the Fish is obviously attained. But it will be observed that, notwithstanding this similarity, the Vertebrated embryo never presents any of those features of the Circulating apparatus, which are characteristic of the other sub-king- doms respectively; thus, it does not exhibit that radiated distribution of the vascular trunks, which is seen in the E chinodermata (§ 216); nor does the heart, even when most like a c dorsal vessel,' ever present the least approach to that transverse division into successive segments, which is typical of the Articulata (§ 217); and in its position and connections, being situated in the immediate neighbourhood of the pharynx, and sending its primitive trunks around it, the heart of every Vertebrated Mollusca opposite extremity of the alimentary canal (§ 234). progress of the Circulating apparatus, from the grade In the subsequent of the Fish, Mammal, we have a characteristic illustration of the principles formerly laid down (§ 74); for although the branchial arches are formed in all Vertebrated animals, yet it is only in Fishes and Batrachian Reptiles that they give origin to branchial tufts ; and although at a subsequent period the condition of the heart and great vessels presents a strong general resemblance to that of the typical Reptiles, yet that resemblance is wanting in the essential feature of the complete separation of the auricles, and the mixture of arterial and venous blood in the single ventricle. It is obvious that this want of conformity has reference to the difference in the seat of the respiratory process ; the pulmonary vessels in the embryo being deve- loped for future use, but the actual aeration of the blood being performed elsewhere. I knowled Circulating apparatus, enables us to explain many of the malformations Man. One of the most common of them gives rise to the malady termed Cyanosis ; for this r esults from the fi remaining open after pulmonary respiration has been established ; so that a considerable portion of the blood transmitted to the right cavity passe* 278 OF THE CIRCULATION OF NUTRITIVE FLUID. into the left, without having been previously arterialised by passage through the lungs. Persons thus affected have always a livid aspect; from the quantity of venous blood circulated through the arteries ; they are deficient in muscular energy and in power of generating heat, and they are seldom long-lived. A consequence partly similar would probably have resulted from a curious malformation mentioned by Kilian, had the infant remained alive : in this case, the aortic arch had not been deve- loped, so wiat tne primary aortic trunk gave off only the vessels t head and upper extremities : whilst the communicating branch be- the pulmonary artery and descending aorta, which usually is of a secondary character, constituting the ductus arteriosus, was here the only means by which the blood could be transmitted to the latter ; so that the circulation through the lower part of the trunk and extremities would have been entirely venous. A malformation of this kind in a diminished degree has not been found incompatible with the continuance of life ; several cases being on record, in which the ductus arteriosus has remained pervious, and has brought part of the blood from the pul- monary artery to the descending aorta. Cyanosis is of course, as in the former instance, the result of this imperfect arterialisation ; and the individual is reduced, as far as his vascular system is concerned, to the condition of the Crocodile. An arrest of development at an earlier period may cause still greater imperfections in the formation of the heart. Thus, the septum of the ventricles is sometimes found incomplete, the communication between the cavities usually occurring in the part which is last formed, and which in most Reptiles remains open. In other cases it has been altogether wanting, although the aorta and pulmonary artery were both present, and arose side by side from the common cavity; and this form of the circulating apparatus is evidently analogous to that presented by Reptiles in general. A still greater degradation in its character has been occasionally evinced ; for several cases are now on record, m which the heart has presented but two cavities, an auricle and a ventricle, thus corresponding with that of the Fish; and in one of these instances the child had lived for seven days, and its functions had been apparently but little disturbed. The occasional entire absence of the heart has already been noticed; and coexistent with this there is always great deficiency in the other organs ; the brain, and sometimes the liver and stomach, being undeveloped. The bifid character of the apex, which presents itself at an early period of the development of the heart, and is permanent in the Dugong, sometimes occurs as a mal- formation in the adult human subject; evidently resulting, like the others which have been mentioned, from an arrest of development. On similar principles, some occasional peculiarities noticed in the distribution of the vessels may be accounted-for, of which a striking example will be presently given. The ascending Cava is occasionally observed to consist of two parallel trunks, which are partially united at intervals, and then separate again ; a similar condition is permanent in some Cartilaginous Fishes, and the explanation of it is to be sought-for in the history of the development of the venous system in general. We have seen that in many of the lower animals, such as the Crustacea, where the arteries are perfect canals, having distinct coats, the veins seem to be merely channels through the tissues, having no definite walls : in like manner, at an early . DEVELOPMENT OF CIRCULATING SYSTEM 279 period of the foetal development of the higher animals, several small vessels are found where one vein subsequently exists ; and, it the coalescence of these has been from any cause checked, they will remain permanently separated to a greater or less extent. < 263 Several interesting varieties have been detected m the arrange- ment of the principal trunks given-off from the Aorta : and though we cannot account for them on the principles already mentioned, it is not > a little curious, that nearly all of these irregular forms possess <™*W™£ the arrangements which are peculiar to some or other of the Mammalia The mode in which the cephalic and brachial vessels usually arise in the Human subiect, is shown in the subjoined diagram, a, where a b is the B C D B P G H a b a b a b a Diagram of the principal varieties in the origin of the Cephahc and Brachial trunks from thS of the Ao?ta ; - A> Man ; B, Elephant; c, Cetacea; n Bat &c ,; | , C— , &c.; f Seal ; G, Buminants H, Eeptiles :-l, right subclavian ; 2, right carotid ; 3, lelt carotid , 4, left subclavian ; 5, vertebral; a, ascending aorta ; b, descending aorta. arch of the aorta, i and 2 the trunks of the right parotid (which ^supplies the head) and of the right subclavian (which is distributed to the upper extremity arising by a common trunk-the ^^^^{^l the left carotid, 3, and the left subehman, 4, arose separately see. a distribution whieh is rare £ *.ta-J«^^ trunk arising Dy a coinmuii n uujv, <*-lxv* ^^ — & ~~ ~~ being given off separately; this is the regular arrangement of branches in the Elephant. It is not so unusual for all the branches to arise from single Trunks a>» w y; > <*^^ "i**» ^rr — - ~ . ^. . , ^^ ^_ v the Cetacea. Sometimes, again, there is an arteria in nominat .on each side ; subsequently dividing into the carotid and subclavian as at n and on this plan the branches are distributed m the Bat ^ e ^ d al ° ™ the Porpoise. A not unfrequent 7^^/ n . the /^inS tmnk a S both carotids to arise with the right *^™J * 7^^!^ th^ at E, the left subclavian coming off by itself; this is observable as the regular form among many animals, being common among the Monkey tribe, the Carnivorat the Rodentia, Ac. Another variety which is not infrequent is shown at E, the vertebral artery on the left side, S , which ZJj arises from the subclavian, springing directly from the aorta ; it I on this plan that the branches are given off in the Seal. A form whicl is ver> uncommon in man is that -Presented at a; here Jhe aoria divides at once into an ascending vessel, from which the two ^clavian and two carotid arteries arise, and a ^^^^^'J^^^Z distribution of the vessels in Ruminating animals and appear^ to be most Mammali most general m mammalia, yv^™^ s - — - ^w™^ seen a* form which evidently results from an arrest of the usual changes . , , .n i • ee oKa o.Kft- +Tip. aorta contmuino' to §§ . H » D f •>:■ ■ . 280 OF RESPIKATION. possess a double arch, from the ascending part of which the subclavian, external carotid, and internal carotid arteries are given-off on each side, the single descending trunk being formed by the union of the two original branches. This, it will be recollected, is the normal type of formation m Reptiles. * CHAPTER VI. h I :' OF RESPIRATION. 1. General Considerations. 264. The function of Respiration essentially consists in the evolution of carbonic acid from the fluids of Organised beings, and the absorption of oxygen from the surrounding medium, usually in a nearly equivalent proportion. This process is performed by Plants as well as by Animals ; and it may be regarded as arising out of the same general requirements m both kingdoms, although it answers some special purposes in the latter, which render it more immediately essential to the maintenance of their vital activity, than it seems to be in the former. For we shall hereafter a r K ^ im P erious necessity for the continual introduction of oxygen and liberation of carbonic acid, which requires a most active performance oi the_ respiratory function, and causes even a brief suspension of it to be total, in the higher Animals, is consequent upon the energetic exertion oi their peculiarly animal powers, and upon the performance of that combustive operation by which their high temperature is maintained : whilst on the other hand, when we pass to those tribes which are most remarkable for the inertness of their habits, and for their entire want of power to sustain an independent temperature, the demand for oxygen is greatly diminished, and the exhalation of carbonic acid may be checked for a time without injury. The amount of Respiration, then, which is required for the performance of the organic or constructive functions of Animals is comparatively small ; and it is not surprising that the exist- ence of this function should have been long overlooked in Plants, in which its effects on the atmosphere are masked by a change of an entirely op- posite nature, that is subservient to the introduction of alimentary material into the system, — namely, the decomposition of the carbonic acid of the air, under the influence of light, the fixation of its carbon in the vegetable tissues, and the consequent liberation of its oxygen. To this last pro- cess, the term Respiration has been commonly applied ; and the Respira- tion of Plants is ordinarily spoken-of as antagonistic to that of Animals. This statement is perfectly true, if under the term Respiration be in- cluded the sum-total of the changes produced in the air by the growth of a Plant ; but it will be presently shown, that whilst Animal life gives * In the foregoing account of the development of the Vascular system, the Author has avaxled himself freely of the valuable papers of Dr. Allen Thomson, in the < ' Edinb. Philos. Journal, Vols. ix. and x ; in the s ketch t of "the malformations of the Heart, he has made use of the paper of Dr Paget m the Edinb. Med. and Surg. Journal," Vol. xxxvi. ; and the fflr^T^m^ the a^P^fg fi g u res, has been entirely derived from the magni- ncent work of Tiedemann on the "Anatomy of the Arteries. " . ^^^1 RESPIRATION IN GENERAL 281 rise to but one set of changes in the atmosphere (namely, the removal of a portion of its oxygen, and a replacement of this by carbonic acid) V ege- table life produces two sets of changes, which ought to be kept quite distinct from each other in a scientific description of them, their nature and their sources being alike different ; and that it is only on account of the excess of one set of these changes beyond that which it antagonises, that it alone has received general attention, and has been commonly regarded as the proper respiration of Plants. 265 Restricting the meaning of the term Respiration, then, to tne removal of carbonic acid from the living system in a gaseous form and the introduction of oxygen into it, we have to enquire what are those most general sources of demand for this action m the vital economy, which are common to Plants and Animals. These appear to be two-fold; one arising out of the disintegrating changes which are always going-on in the living system : the other being consequent upon some of those chemical operations, which necessarily participate m the constructive Wtions The former seem to be the most general; the latter are rather of a special character, and manifest themselves most strongly as we shall see hereafter, at particular periods in Vegetable life (§ 2 7 4). -All organised bodies, as alreadv explained, are liable to continual disintegration even mm£ in fact, a succession of organs whose individual duration is short, but whose functional energy is great, seems necessary for the maintenance oi the life of the more permanent parts of the organism (chap, hi., beet. ^1). The n ecessary result of this disintegration is decay ; and one of the chiei products of that decay is carbonic acid. A large quantity of this gas is set free during the decomposition of almost every kind of organised matter, the carbon of the substance being united with oxygen supplied b V the air. Hence we find that the formation and liberation of carbonic acid go-on with great rapidity after death, both in the Plant and m the Animal ; its disengagement being but the continuation (so to speak) ot that which has been taking place during life. Thus in Plants, so soon as they become unhealthy, the extrication of carbon in the form of carbonic acid takes place in greater amount than its fixation from the carbonic acid of the atmosphere ; and the same change normally occurs during tne period that precedes the exuviation of the leaves, their tissue bemgno longer able to perform its characteristic functions, and its incipient decay givfng rise to a large increase in the quantity of carbonic acid set free. In some of these cLs, it would seem that the carbon of the decomposing tissue unites with the oxygen contained in the fluids of the system ami that carbonic acid is thus generated, the extrication of which contributes to the introduction of fresh oxygen (§ 266) : in other instances, however, the oxygen may be more directly derived from the atmosphere.— The other source of demand for Respiration, which is common alike to Plants and to Animals, arises out of the chemical transformations which are always going-on in their systems, as a part of their nutrient operations. These are as yet but very little understood; but enough * known to justify the belief, that in many of them the presence of oxygen is essential, and that carbonic acid is among their products. Examples of such trans- formations, drawn from the Vegetable kingdom will be given hereafter (chap, vin., Sect. 2); but it may be remarked m this place, that the £tA- 282 OF BESPIRATION. conversion of starch into sugar, a change that takes place in the neigh- bourhood of many growing parts, is accompanied by the combination of carbon with oxygen to a considerable amount ; and that, in general, tlie production of the vast multitude of organic compounds yielded by Plants, from the substances which are first generated by them at the expense of +.T)A ir>m*o - anir> a!otyiot>+o »^n>i,'™„„ ~ • t> i • i ■> • t chemical Although of which oxygen is taken-in and carbonic acid eiven forth. the number of organic compounds generated I than that which we find in Plants, yet there can be nodoubt, from "a comparison of their atomic constitution, that oxygen must be taken into combination, and carbonic acid given-off, in many of the chemical trans- formations which take place in the living body; some of the most remark- ■Besides the evolution of carbonic acid and the absorption of oxygen, it would appear that the exposure of the circulating fluid to the air is the means of keeping the Nitrogen of the system at its proper standard; this gas being absorbed or exhaled, according as there is a deficiency or a superfluity of it in the fluids of the body (§ 320). 266. The whole series of reactions taking place between the living organism, and the air which surrounds it or which is contained in the water wherein it lives, may be conveniently included under the general term Aeration. This aeration would appear to be, like absorption, a change dependent on physical agencies, and occurring in conformity with their laws, when the requisite conditions are supplied by the structures of an organised being, and by the functional alterations which the living state involves. — All gases of different densities, which are not disposed to unite chemically with one another, have a strong tendency to mutual admixture. Thus, if a vessel be partly filled with hydrogen, and partly with carbonic acid, the latter, which is 22 times heavier than the former, will not remain at the bottom, but the two gases will be found in a short time to have uniformly and equably mixed; and it is on this principle ywh the gases which compose it are of different specific gravities. So strong will take place when a membrane or other porous medium is interposed between them. This interchange, therefore, evidently resembles the § cause the nature of the septum has so much influence over the phenomenon as sometimes to reverse the results. "When plaster-of-paris is employed as the medium of diffusion, the exchange will take place with simple relation to the relative densities of the gases ; and a general law has been ascer- tained by Prof. Graham, which applies to all instances, — that the ' replac- ing' or ' mutual-diffusion' volumes of different gases vary inversely as the square-roots of their densities. Thus, if a tube, closed at one end with a plug of plaster-of-paris, be filled with hydrogen, the gas will soon be entirely removed, and will be replaced by something more than one fourth of its bulk of atmospheric air ; the density of hydrogen being about 1-1 4th that of the atmosphere. "~ ployed, the result is much influenced by the relative facility with which each gas permeates the septum. But when organic membranes are em- •bonic / • RESPIRATION IN PLANTS 283 ;: bladder much more readily than hydro F does; ^ k «q«e, wh PTi a bladder of hydrogen is placed m an atmosphere of carbonic acid, a certain quantity of hydrogen will pass out j but a much larger proportion o! carbo^c acid 7 will enter, so as to distend the bladder even to bursting Further it is found that, if a fluid be charged with any gas which it r eaclilv absorbs (as, for example, water with carbonic acid), it will speedily Zt 4hTwhen exposed to the attracting influence of another gas, such LttmospherTc air; and the more different the densities of the two gases th e more rapidly , and with more force, will this take place. As m the a porous membrane; and part of theextenor gas will be absorbed bythefluid , 4 \ , va ± n -u' , n imbibedV in place of that which has been removed. IZTJ ffi ifexMeTand is replaced b y absorbed ^ ; and S ^exhalation and absorption of nitrogen take plaee m animals, and perhaps also in plants. 2. Respiration in Plcmts 268 Under the above designation have been associated two distinct n ,1 „i~ ~ „+«^+ +hvAn^mif, f.lift Vegetable kingdom. Ine changes, uuwi ucanj wixou«**v — — ^ — -- — o . - . atmosphere being the chief source whence Carbon is supplied to the livingW, the introduction of that element , ta .been £*«**»* necessary h °ZSe anS VhS' co^sponds exactly with the respiration of Animals. The introduction of carbon is effected by the power which the green surfaces of Plants possess, of decomposing, under the stimulus ot fight, the carbonic acid contained m the am or mjtoh^* supplmd to them; and of retaining or fixing its cart hil surfaces S append^ tTtleTS arl those by which this fixation^ carboy which may be considered as a process of alimentation, is chiefly, if not entirely effected ; and where, as in the Cactus tribe, the leaves ^effi- cient, but the stems are succulent and their surfaces green, t is ob vious that ihese last perform the same functiom In the *™>*^£j^ is the same separation of parts as in the Flowering P^^^ *^ is here also, without doubt, performed by the green ^parts , of the surface Of the inferior Cryptogamia, however, we know J^ 1 ^ m rf ^ tS would not seem to depend upon the atmosphere for any part ot their St of carbon, which is altogether furnished by their peculiar aliment (U21); ^d these plants scarcely ever present any green surface, and flourish most in situations to which light has but little access The same maTbe said of the Guscuta (dodder) and other leafless parasitic plants of ml complex structure, that live upon the prepared juices they derive Lm the plant to which they attach themselves^ There can be™ doubt that Lichens ordinarily obtain the carbon which enters mtojhen ^ B w ture, entirely from the atmosphere ; and, that the ^ u ^X Jwater like manner by the carbonic acid contained m the circumambient water, u*.e iiidiimei , uy wi 00 „ -H-ai-n the nrecise conditions under but experiments are yet wanting to asceitam the prco surfnoos . which its assimilation is effected. Few Lichens have any green surtaces, I \ ■ I 1 H 284 OF RESPIRATION, and although many of the Algse are very brilliantly coloured, yet we find them occasionally existing at such depths, as make it difficult to believe that light is the only stimulus under which they can attain this appear- ance. Confe Thus the floating islands fresh water, appear to exercise an important influence in maintaining it in a state fit for the support of animal life ; since it seems probable that they absorb the products of the decomposition of that foul matter by which all ponds and streams are constantly being polluted, and at the same time yield a supply of oxygen to the water. It is a notorious fact that Fishes are never so healthy in reservoirs destitute of aquatic plants, as in ponds and streams in which these abound. 269. The entire mass of Vegetation upon the surface of the globe, _._ thus mainly dependent upon the minute proportion of carbonic acid con- tained in the Atmosphere, which is not above 5 parts in 10,000. This seems to be as much as Plants in general, under the feeble illumination which those of them are liable to receive, whose ' habitat ' is in variable climates, could advantageously make use of; and a larger proportion would probably have been injurious to them, as well as to Animals. But it has been ascertained by direct experiment, that Plants will thrive in an atmosphere containing six or eight per cent, of carbonic acid, or even more, so long as they are exposed to strong sun-light ; and it would appear that in climates where the solar light is less obscured by clouds than it is in our own, the growth of plants may be favoured by an unusual supply of this alimentary substance. which are constantly being formed on the lake Solfatara in Ttaly^ exhibit a striking example of the luxuriance of cryptogamic vegetation in an atmosphere impregnated with carbonic acid. These islands consist chiefly of Confervse and other simple cellular plants, which are copiously supplied with nutriment by the carbonic acid that is constantly escaping from the bottom of the lake, with a violence which gives to the water an appearance of ebullition.* Dr. Schleiden mentions that the vegetation around the springs m the valley of Gottingen, which abound in carbonic acid, is very rich and luxuriant ; appearing several weeks earlier in spring, and continuing much later in autumn, than at other spots in the same district, t — A. very ingenious hypothesis has been raised by M. Brongniart upon the fact, that an increased quantity of carbon may, under particular be assimilated by Vegetables. He supposes that, at the epoch of the growth of those enormous primeval forests which supplied the materials of the coal-formation, the atmosphere was highly charged with carbonic acid, as well as with humidity ; and that from this source the Ferns, Lycopodiacese, and Coniferse of that era were enabled to attain their gigantic development. He imagines that they not only thus con- verted into organised products an immense amount of carbonic acid, which had been previously liberated by some changes in the mineral world, but that, by removing it from the atmosphere, they prepared the earth for the residence of the higher classes of Animals. The hypothesis is a very interesting one, and well deserves consideration. It may be regarded as an almost absolute certainty, that the whole of the carbon now solidi- fied in the coal-deposits of various ages, must have previously existed in * Sir H. Davy's " Consolations in Travel," 3rd ed. p. 116. f " Wiegman's Archiv." Bd. iii. 1838. circumstances RESPIRATION IN PLANTS. 285 the- atmosphere; and if we were acquainted with the extent of these it would be a simple matter of computation to determine, whether, if all this carbon were reconverted into carbonic acid, it would sensibly atiect the proportion of that ingredient in the atmosphere.— The recent experiments f~n T> "hnV^^" /Mv«fcQji-n *w+.rOIl- t 01 jur. xjzuuvny uu - «. ,-—„ — hypothesis of M ** ***** ***** ^ cons sts in the disengagement of the superfluous carbon of the system, either by combinatkm with the oxygen of the air, or (which is more Ekebri by replacing with carbonic acid the oxygen that has been absorbed from it ; and it does not cease by day, by night, in sunshine, or m shade. If the function be checked, the plant soon dies— as when placed m an atmosphere with a large proportion of carbonic acid and without the stimulus of light which enables it to decompose the deleterious gas. Plants which are being < etiolated ' by the want of light, absolutely diminish in the weight of their solid contents, owing to the continued excretion of carbon by the respiratory process, although their bulk may be much increased by the absorption of water ; and if the proportion of carbonic acid in the surrounding air be augmented by its accumulation, they become sickly and die, from the impediment to their respiration. The parallel, therefore, between Plants and Animals appears to be com- pletef as regards the influence of carbon upon their growth; for to both it is deleterious when breathed, while to both it is invigorating when intro- duced through the digestive system as food; and whenever P ants, or parts of Plants derive their nutriment, like Animals, from organic compounds ii^Cred for them, there do we find the true respiratory process * « Report of the British Association" for 18« » ; p. .66. whether there + The Author would suggest it as a point worthy tf^^f* ^ ' lants that fur . m ay not have been a special relation Mj»to 1™™£ K ttercalated amongst the nish those carbonaceous deposxts, wtic ^ e ^^ ^Eions ■ and the deposit of those vast Carboniferous Ool tic , Wealden^ jj^ ™Sed. The latter, there is strong calcareous beds with which they are so rem ar* * jr _ car bonate of lime having been reason to believe, are almost entirely of ~l g from the wTters of the ocean, just as the drawn by Zoophytes , Echinoderms and ™^S^^^,M o'f the atmo- carbon of the vegetation of these per Msw i araw deposit s, there was an unusual snhere Now if we imagine that during the progress ol tnose aepowu^ • ST)rill „„ spnere. in own m > i g solution by free carbonic acid, by submarine springs discharge of carbonate ol lime, neia . lu /1 & ." lu " q„i.p a+nT . a niir i other calcareous springs, which issuing from the interior of the earth (like the ^£^T^ growth of plants at the furnish the ' travertine' deposits, and at the same time p ^°^Xf^ unda P nce of ' carbo . present day), we seem to have a probable account < of _ the e^traOTdmary nate of lime in the ocean-waters, and of ^™ *"*£ ^o Ihe Sr, so soon as it became since the greater part of the latter would have escaped into tne ,^ free to do so by the removal of the pressure which prevwusly restrained 286 OF RESPIRATION. I J :: U taking place, without the counterbalancing fixation of carbon as aliment. This is the case, for example, with Fungi in general ; it is the case, too, with the leafless Phanerogamic parasites, which draw their materials from the elaborated j uices of other plants, instead of preparing them for them- selves (§ 352) ; it is the case also with the growing embryo, whose food is derived from the store laid-up in the seed, and whose action upon the air is contrary to that of the developed plant, until it has exhausted this store, and has unfolded its leaves to the light so as to take in fresh carbon from the atmosphere ; it is the case, again, with all the growing parts which derive their nutriment from the leaves (not being themselves able to decompose carbonic acid), and especially with the flowers ; and it is the case, too,_ even with the leaves themselves, when their functional activity is diminishing, and the decomposing changes in their tissue are com- mencing. 271. It becomes a question of much interest, to determine the relative amounts of carbon thus absorbed and excreted by Vegetables. Since a large part of the solid material of their tissues is derived from the atmosphere, it is evident that the whole quantity of carbonic acid in the air must be diminished by their growth ; but as a certain proportion of that carbonic acid is taken-in by the roots, which are supplied with it through the absorbent agency of the soil requires to be tested by experiment. very Such experiments have been * general result of them is, that, so long as the leaves continue in healthy action, and are exposed to the influence of light, they are actively engaged in taking-in carbon from the atmosphere;* and that, when entire plants, consisting not only of leaves, but of stems and other parts, are confined in the same portion of air, day and night, and are duly supplied with carbonic acid gas during sunshine, they will go on adding to the proportion of oxygen present, so long as they continue healthy; the slight diminution of oxygen and increase of carbonic acid which take place during the night, bearing no considerable ratio to the degree in which the opposite effect occurs by day.f The balance of nutrition, therefore, between the Animal and Vegetable kingdoms, is thus main- tained in a very perfect and interesting manner. 272. With regard to the char^ principal constituent of the atmosphere — Nitrogen definite statement can be made. Although this element enters largely into the composition of Plants, there seems reason to believe that all which they require is derived by them, not directly from the atmo- sphere, but by the decomposition of the ammonia absorbed by the soil, no very certain or that the free nitrogen of the air has any concern with vegetable respiration ; for the few experiments which have been performed with express view to this subject, lead to the belief that azote is as frequently exhaled as absorbed. 273. In the Fungi among Cryptogamia, however, and in the 'leafless * See the experiments of Mr. Pepys in the " Philosophical Transactions' ' for 1843, p. 329. t See Dr. Daubeny's letter to Prof. Lindley, in his "Introduction to Botany " Vol. ii. p. 297. J? ^^ ANTS 287 mrasites' (such as the tribe of Orohanchece) among Phanerogamia we SyTexampL of the performance of the true respiratory process without fh rantaSin- action of the alimentative ; the only change which the ^^rfSWte induces in the surrounding air being the replace- f.^ of its oxv^en by carbonic acid. Thus, from the experiments of ment ol its oxygen yy „„„:„' a ^ ft a absorbed from the Marcet ^l^Uofo^jgen-, a portion of which appears to combine ^^A g C ^the St, and thus to form the carbonic acid which "^tfit wMls^ th remainder seems to be retained in its structure, replaces it, whilst tne rei rrmArvtian of the Oroban- peculiarly j\jL. XJvyij ^.j^w— ~- j. -- He found that in every stage of their t^JTZ ESrfSS plan ts-hethev they are expesed te aota vegetation, «*u f ^ ^ dark ab sor b oxygen and give out light, or ^f^ & l^fZ car bonic' acid generated being nearly carbonic acid the v^ ^ ^ ^ production f equal to that /^ e o fJ g r Tsuh T of a mere union of carbon excreted by the carbonic acid is not a re ^ fe t ^ lace in the interior of the plant, with atmospher , oxygen, but ta p^ ^ & ^ £"? 2 rt^^SS^ continuesfor a time when the plants £ SSJS^tT^oBp^ of pure hydrogem (Aparallel fact wd hereafter be cited in regard to the respiration of Animals, § 3190 The amount of carbonic acid exhaled is augmented by warmth, which in- creases the activity of the nutrient operations j and as m other plants, it Tpectl aSy grea/during the period of flowering. The contrast between the action of one of thest leafless parasites upon the atmosphere, and that of the nlants from which they draw their nutriment, was curiously dis- p Wd tltT- two receivers of the same capac ty, a portion of the stem of L OrobLhe, and a portion of the leafy stem otthe Teucrmm on wh ch it grew, each piecebeing of the same weight; the atmosphere surrounding them was composed of six volumes of common air mingled with Tone of carbonic acid; and they were exposed to light from 9 a.m. until 3 M of the succeeding day. At the end of this tune, the atmo- rihere of the jar containing the Teucrium did not present a trace of carbonic acki whilst that in which the Orobanche had been immersed exSbfted^uch an augmentation of carbonic acid, that whilst its original SSS»i X^S^t thTtr been %&!%£&% ^^-^SS^^T^^^ Stute i the power 5 EJpolg th Z^^v^^tetoVO^ oi ^alimenta- tion S the sole change which they produce m the surrounding air, is of Sie ie kind with the respiration of animals. As already remarked tW is ^very reason to believe that the same change takes place m all there is eveij *<** oontinuallv £iven-off from their other Plants, and that ™}*™>»Z*™ °lZu2 lit is masked by the interior, whilst oxygen i absc rbed adthou h^ ^ ^ opposite change, which is enectett oy y ^ ^ influence of light upon them. ^ r ~ ^^^Xt ; and it has been the latter, the respiratory change makes itsell manner , * U Annales des Science's Naturelles," 3* Ser., Botan., Tom viii., p. 158. u 288 OF RESPIRATION. shown by the experiments of Saussure, that through the dark portions of plants, this change is continually taking place. 274. Further, there are certain processes in the life of all Phanero- gamia, in which the function of Respiration seems to go on with remark- able activity, and in which its manifestation is not concealed by the converse operation. One of these is Germination, or the development of the young plant from seed, which requires that the starch laid-up by the parent for the support of the embryo should be converted into sugar, the latter being the form in which it is applied to the purposes of nutri- tion. This conversion involves the liberation of a quantity of carbon, which is disengaged by means of its combination with the oxygen of the surrounding atmosphere ; and the young plant may then be regarded as living under the same conditions as the parasitic tribes just referred-to its nutriment being supplied to it without the necessity for the alimenta- tive operation which it will be afterwards required to perform. Germi- nation takes place most readily in the dark, since the extrication of carbon, which is the most essential part of the change, would be anta- gonised by the influence of light. The young plant is, therefore, much in the condition of one which is being ' etiolated ;' and it is accordingly found that, during the early period of germination, the weight of the solid contents of the seed diminishes considerably, though its bulk in- creases by the absorption of moisture. This is its state until the cotyle- dons, or seed-leaves, have arrived at the surface, and temporarily perform the functions of leaves. It is an interesting fact that, after many trials, germination has been found to take place most readily in an atmosphere consisting of 1 part oxygen and 3 parts nitrogen, which is nearly the proportion of the air we breathe. If the quantity of oxygen be much increased, the carbon of the ovule is abstracted too rapidly, and the young plant is feeble ; if the proportion be too small, carbon is not lost m sufficient quantity, and the young plant is scarcely capable of bein roused into life.—The changes which take place during Flowering, are very similar to those occurring in germination. A large quantity of oxygen is converted into carbonic acid by the action of the flower ; and it is believed that the starch, previously contained in the disk or recep- tacle, is changed by this process into saccharine matter adapted for the nutrition of the pollen and young ovules, the superfluous portion flowin g off in the form of honey. analogy g germination and flowering holds good, not only in their products but in the conditions essential to their activity. Neither will commence except in a moderately warm temperature ; both require moisture for flowers will not open unless well supplied with ascending sap • and' the presence of oxygen is in each case necessary. It has been well ascer- tained that the carbonisation of the air bears a direct relation to the development of the glandular disk, and that it is principally effected by the essential parts of the flower, or organs of fructification. Thus, Saussure found that the Arum Italicum, whilst in bud consumed in twenty-four hours 5 or 6 times its own volume of ■ oxygen ; during the expansion of the flower, 3 times ; and during its withering, 5 times. When the floral envelopes were removed, the quantity of oxygen con- sumed by the remaining parts was much greater in proportion to their volume. In one instance, the sexual apparatus of the Arum Italicum ac- BESPIRATION IN PLANTS 289 consumed in twenty-four hours 132 times its bulk of oxygen. Saussure also observed that double flowers, in which petals replace sexual organs, vitiate the air much less than single flowers in which the sexual _ organs ' § 365).— The same is the case, again, during the development of leaf-buds from parts m which as m the tuber of the Potato) a supply of starchy matter has been laid up as the material for their evolution/until they are so far expanded that they can obtain carbon from the atmosphere. Here, too, the growing bud is m the condition of a narasite beine supported upon materials provided for it by other agencies E til own? Inhere, again, we find that the conversion of the starch into sugar is the process with which the production of carbonic acid aP 27r MtoSJ mTanT of aeration which the transmission of the nutritive fluid to the external surface affords, the more highly organised Hants seem to have the power of admitting air into cavities «»£* £ suSvient to this purpose, will be hereafter described (§§ 325, 326) under the head of exhalation, for which function ft appears more parti- cularly designed. But, superadded to this, we find in thePhanerogamia a system of tubes apparently intended to connect the interior of the structure with the external air. These are the spiral vessels, which, in their perfect form, are seldom found to contain any but gaseous fluids. In Exogens they are usually confined to the < medullary sheath imme- diately surrounding the pith ; in Endogens they are ™™™™ersatty distributed through the stem, forming part of every bundle of fibro- vascular tissue. In each case, however, they traverse the stem m such a manner as to enter the leaves through their footstates ; and they seem to communicate with the intercellular passages, and, through their medium, if not more directly (as some have supposed), with the externaf air. A. curious analogy exists between these respiratory tubes and the trachea =of Insects (8 302) : and although their exact office is not fully ascertained, there can be little doubt that they contribute in some way to the aeration of the internal fluids. It has been found that th ey.contem . * largei quantity of oxygen, by 7 or 8 per cent., than that -hichex ,sts m the atmosphere.-In a great number of the aquatic tribes, both ^ng *he simpler and the more highly organised plants, we ^^^^ adapted for the inclusion of air ; but these would seem designed i atner to ?ive buoyancy to the structure, than to take any share in the Respi- ratory function. The air which they contain, however, is seldom identical in composition with that of the atmosphere. . 2 7 6 Regarding the progressive evolution of the Respiratory system m Plants much might here be said, which will perhaps be more *W ^X7Lrel to the account of their general **^*^^ It may be remarked, however, that the early form of the embryo ol the S^ffi^to — bl6S in ltS ^ S SP T^t dSin 1 reSt vegetation of the cellular Cryptogamia al hough it <^^ t ^ *° thf mode in which nutriment is supplied ; the latt er derm ng^ it by the ir unassisted powers from the surrounding el ^nts whilst the former is provided with it by the parent. At the first period of the germination J fl :i ' . 290 OF RESPIRATION. i u of the seed, a close analogy exists, as we have seen, between the embryo and the tribe of Fungi. Both are specially supplied with nutriment^ which is prepared in the one case by its parent, and in the other by the decay of animal or vegetable matter; both are developed most rapidly when supplied with warmth and moisture, and in the absence of light ; and both liberate carbon to a large amount, without assimilating any from the atmosphere. By the time, however, that the cotyledons have risen to the surface and acquired a green colour, the plant has advanced a stage in its growth, and as to its respiratory system has now attained the level of the Marchantia (§ 27), possessing, like it, stomata and inter- cellular spaces, but being destitute of spiral vessels. These do not appear until true leaves are evolved ; and by the time that this last stage in the development has taken place, the cotyledons, which may be regarded as temporary respiratory organs, decay away. — When we have traced the evolution of the respiratory system of Animals in a similar manner, we shall observe a most interesting correspondence between the consecutive phenomena, as they occur in the two kingdoms respectively. 3. Respiration in Animals. 277. The dependence of the life of Animals upon the constant per- formance of the Respiratory function, is more immediate than that of Plants ; and this arises, not merely from the circumstance that there are sources of demand for oxygen and of production of carbonic acid in the former, which do not exist in the latter ; but also from the fact, that from those which are common to both (§ 265), the amount of carbonic acid generated, as well as of oxygen required, is far larger in the Animal than in the Plant. The more active are the organic functions, and the softer and more prone to decomposition are the tissues, the more con- siderable will be that constant decay to which all organised fabrics are exposed, even during life : and thus in < warm-blooded' animals, the high temperature of the body, which favours the vital activity of its com- ponent organs, and causes them to live fast, will accelerate their decay, and will hence give rise to a more rapid production of carbonic acid and to a greater demand for oxygen; whilst in ' cold-blooded' animals so lon/ U/e But when the temperature of the Reptile is raised by external heat nearer the level of that of the Mammal, its need for respiration increases, owing to the augmented waste of its tissues. When, on the other hand the warm blooded Mammal is reduced, in the state of hybernation, to the level of the cold-blooded .Reptile, the waste of its tissues diminishes to such an extent, as to require but a very small exertion of the respiratory process to get rid of the carbonic acid which is one of its chief products. And in those animals which are capable of retaining their vitality when frozen, or when their tissues are completely dried-up, the decomposition is for the time entirely suspended, and consequently there is no carbonic acid to be set free. 278. But another source of Carbonic acid to be set free by the Respi- ratory process, and one which is peculiar to Animals, consists in the rapid changes which take place in the Muscular and Nervous tissues, during the period of their activity. Every development of muscular I MM RESPIRATION IN ANIMALS 291 force or of nervous power is accompanied by a destructive change in a certain amount of tissue, to which change the presence of Oxygen is essential; and one of the proc"— -* "~ — — <* ~ -** «* elements of the Nervous and union Muscular Henc xience 113 may uc duo-v^ ^ ~ & ___ r -.-^^- r -w, — - x these substances, which is a condition of their functional activity, and which is altogether distinct from the general slow decay that is common to them witlfother tissues, is another source of the generation of car- bonic acid and of the demand for oxygen m the animal body; and that the amount of the one gas produced and of the other gas required, will consequently depend upon the degree m which these tissues are exercised In such animals as are chiefly made-up of the organs ot vegetative life, in whose bodies the nervous and muscular tissues form but a very small part, and in whose tranquil plant-like existence there is but very little demand upon the exerme of their functions the quan- tity of carbonic acid thus liberated will be extremely small, and the dependence upon a supply of oxygen by no means close. On the other hand in animals whose bodies are chiefly composed of muscle, and whose life is an almost ceaseless round of exertion, the quantity of carbonic acid thus liberated is very considerable, and the demand for oxygen is incessant : so that vital activity is speedily suspended, if the respiratory function be not performed.— We are enabled to trace the connection between the amount of nervo-muscular exertion, and the energetic per- formance of the act of respiration, in the class of Insects, better than m any other. They have no fixed temperature to maintain ; and they are consequently not in the condition of warm-blooded animals, m which the quantity of carbonic acid set-free is kept-up to a more regular standard by the provision to be presently noticed. On the other hand, they are pre-eminent among all animals in the energy of their muscular power, as related to the bulk of their bodies, so that the waste of muscular tissue during their state of activity must be very great; and we shall hereafter find that the amount of carbonic acid generated m a given time bears a close correspondence with this (§ 6U). Amon § Reptiles, also, the degree of nervo-muscular activity as of the vital rature of the body; so that the demand for respiration from both sources _- — m T II it v #1 *J \ 2T9rBerides" these sources of demand for Respiration, which are common to all Animals, there is another, which appears to be peculiar to the two highest classes, Birds and Mammals. These are capable ot maintaining a constantly - elevated temperature, so long as they are sumuied with a proper amount of appropriate food ; and their power of doing so (save when the oxygenation of the < waste of the tissues is of itself sufficient to generate the requisite amount of heat) is dependent upon the direct combination of certain elements of the food with the oxygen of the air, by a process analogous to combustion (§ 129). The quantity of carbonic acid that is thus generated, seems to vary consider- ably in different animals, and in different states of the same individual : but 7 the principal source' of difference lies in variations of external tem- perature ; for the energy of the Respiration increases with its diminution, Lee more heat must then be generated ; and dvmmshes when an increase i ■ 292 I OF RESPIRATION. of external warmth renders the production of heat within the body less necessary to sustain its temperature. Whenever the temperature of the surrounding medium is below that of the body, if a sufficient supply of food be not furnished and the store of fat be exhausted, the animal dies of cold (§ 116). 280. To recapitulate, then; the sources of the production of Carbonic acid^and of the demand for Oxygen, in the Animal body, are fourfold.— 1. The continual decay of the tissues, which is common to all living organised bodies ; which is retarded by cold and dryness, and accelerated by warmth and moisture ; which takes place with increased rapidity at the approach of death, whether this affect the body at large, or only an individual part; and which goes on unchecked when the actions of nutrition —2. The various changes in composition that take place in the fluids and tissues in the progress of their organic construction many of which involve a liberation of carbon and a higher oxygenation. h^ve ceased altogether. destructive Muscular direct relation to the degree in which they are exerted. — 4. The direct conversion of the hydro-carbonaceous materials of the food into carbonic acid ; which is peculiar to warm-blooded animals ; and which varies in quantity, in accordance with the amount of heat to be generated. 281. The process of aeration is accomplished, chiefly if not entirely, through the medium of the fluids of the body ; but the nature of the fluid which is subservient to this purpose varies greatly in different classes of animals, as does also the method employed. We find, indeed, in many instances, that no special instrumentality is required ; the aeration of the fluids being sufficiently provided-for, by their exposure to the surround- ing medium through the thin membrane which forms their external integument, or through its prolongation into their internal cavities ; and the renewal of the stratum of that medium in contact with their surface, being accomplished by actions that are more directly subservient to other purposes in the economy.— When the special organs appropriated to the performance of this function in the principal tribes of the Animal kingdom, are brought into comparison with each other, they appear at first sight so very different, that a superficial observer would hardly trace any relation between them (§§ 8, 109). A little reflection, how- ever, will show, that all their forms are reducible to the simple element of which the respiratory organs are constructed throughout the Vege- table kingdom ; namely, an extension of the external surface, peculiarly adapted, by its permeability to gases, for the interchange of ingredients between the circulating fluid spread out on one side of it, and the aerating medium which is in contact with the other. Considered, there- fore, under this 6 instrumental' character, there is a complete ' unity' amongst them all; but when considered with reference to the general plan of structure, we find them to be i homologically' diverse. Thus, the extension may take place from very different parts of. the surface, so that its connections with other organs may be altogether dissimilar in the several classes of Animals. Again, it usually takes place internally or externally, according as the animal is to be an inhabitant of the air or of the waters. In animals modified for atmospheric respiration, the air enters the system to meet the blood : a peculiar set of movements, • pr_ _ i KESPIRATION IN ANIMALS. 293 more or less complicated, being appointed for its constant renewal by successive inhalation and expulsion. In those adapted to an aquatic residence a different plan is usually followed. The small quantity ot air contained in the water, is all that the respiratory system employs; and it would have been a useless expenditure of muscular exertion, to have provided means for the constant inspiration and expiration of a lar-e amount of so dense a fluid. In all the higher aquatic animals, therefore the aerating surface is extended outwardly, instead of being prolonged inwards ; and the bl ood is propelled through it so as to come into relation with the surrounding medium, the portion of which m apposition with it is continually being renewed, either by the natural movements of the animal, or by others more expressly contrived for the purpose In the higher Radiata and the lower Articulata, however, we meet with a peculiar apparatus for introducing water from without into the interior of the body, and for causing it to perform a kind of circu- lation through a system of canals more or ess extensively distributed ; the apparatus subservient to this operation has been recently designated the aquiferous or water-vascular system.— The relation between some of the most diverse forms of these organs will, perhaps, be made more apparent by a simple diagram. Let A B represent the general external surface of the body; then at a is shown the character of a simple out- ward extension of it, forming a foliaceous gill, such as is seen m the lower Crustacea ; and in like manner, b may represent a simple internal prolongation or reflexion, such as that which forms the pulmonary sac of B Diagram illustrating different forms of Respiratory Apparatus :-a, simple leaf-like gill; b, simple Diagram musir^ ^ _ ^ ^^ ^ _ ^ ^^ sac . ^ pulmonary hl&uc ^ c f S pl der. the air-breathing Gasteropods. A higher form of the branchial apparatus is shown at c, the respiratory surface being extended by the subdivision of the -ill into minute folds or filaments, as we see in Fishes; and a more elevated form of the pulmonary apparatus is seen at d, the membranous surface being extended by subdivision of the internal cavity, as we find to be the case especially in Birds and Mammals. Lastly at e is shown a i of one of the ' pulmonary branchial of the Arachnida, which forms kind of transition between the two sets of organs ; the extent of sur- f beine -iven by gill-like plications of the membrane lining the inte- ^ Ce f a pulmonic cavity. — Putting aside such modifications, however, as aT destined to suit the particular conditions under which the function is to be performed, and looking simply at the essential characters of shall observe, on tracing them upwards the Respiratory organs, we .,---, ~ A o1 " • v through the principal classes of animals, the same gradual specialisa- tion which has been noticed in the other systems; for, beginning with the lowest it will be seen that the general surface is the organ of 294 I I I OF RESPIRATION. respiration as well as of other functions; whilst, in the highest, the aeration of the blood is almost entirely effected in one central apparatus adapted to it alone, although the general surface is not altogether destitute of participation in it. 282. In the simpler Protozoa, there is no other fluid to be aerated than that which is contained in each independent cell ; and this stands in the same direct relation to the water-atmosphere which bathes its exterior, as is borne by the fluid-contents of the component cells of the highest organism, to the blood-current which brings to them the oxygen they require and carries-off their excrementitious carbonic acid. It may be presumed, on the general principles already stated, that the active Infusoria will require a greater amount of respiratory change than the sluggish Rhozopoda; and this is provided for them by the very instrumentality which confers upon them their peculiar activity, namely, the ciliary action, which either propels them through the water they inhabit, or produces currents in the fluid in immediate contact with their bodies. This same instrumentality,* in the composite Sponges, maintains a current in the system of ramifying canals that extends through their interior ; a system in which we may be said to have the first indication of a gastro-vascular cavity, a ' water- vascular' or i aquiferous' system, and a circulating apparatus, not yet differentiated from each other. The fluid which circulates in the Sponge is obviously sea-water, holding in suspension or in solution those minute particles, at whose expense the growth of the component cells of the organism takes place ; but it evidently serves also for the aeration of their contents, and also conveys away their excrementitious products. 283. The provision made in Zoophytes and Acalephce for the perfor- mance of the respiratory function, is not essentially elevated above that which suffices in Sponges. We have seen that these animals possess no other circulating fluid than the chymous product of digestion, which, mingled with sea-water, is transmitted into the prolongations of the polype-stomachs through the absorbent stem and branches of the Hydra- form Zoophytes, into the perigastric spaces of the Actiniform and Alcy- onian polypes (and their extensions through the composite fabrics of which they form part), and into the gastro-vascular canals of the Acalephse. This ' chymaqueous fluid,' as it may be termed, will serve to aerate the fluids contained in the tissues of the interior of the body, when it is freshly introduced from without ; but if it be long retained within these cavities, it will itself require fresh aeration. It is kept in continual movement within them, by the action of the cilia which usually clothe their surfaces ; and it is not improbable that the flux and reflux of this fluid which has been observed within the tentacula of Actiniae t like the movement which may be perceived in the contents of the gastro-vascular canals that are distributed near the margin of the disk of Medusse is especially destined for its aeration, by exposing it to the circumambient water through a thin intervening septum. It seems not improbable, more- over, that the orifices which certainly exist at the points of the tentacles * See the observations of Mr. Bowerbank on the Ciliary movement in Grantia compressa, in " Transact, of Microsc. Soc.," Vol. III., p. 137. and those of Dr. Dobie, in "Groodsir's Annals of Physiology," No. II. f See Dr. Sharpey's account of this movement, in Art. < Cilia,' " Cyclop, of Anat. and Physiol.," Vol. L, p. 614. RESPIEAT10N IN ECHINODERMATA 295 of manv species (at least) of Actiniform polypes,* and the (so-called) anal ol many species \ , M^n^. mav serve to introduce fresh =fe r^Xi ^o^^dl^V; W to that which has already WmTeBete thus foreshadowing the special water-vascular system of hiXr animSs. In some species of Actinia, indeed, the integument of other pS Tap^Tto he fenestrated so that the animal can Uke-m or eject w«+Pr through apertures which it has the power of opening or closing , rftht can scaLly he for any other purpose than that of respi- ra 1°S4 f The new relations which the digestive apparatus acquires in jb4. J-ne new ( x cavity of the body/ involve, as we have Echmodmvnateto ^ 8^L bet ; een the cliymous contents of the former anal: < chytqueous fluid of the perigastric cavity, which here iormer ana tne ?V*H j , th piir po S es which are answered appears to serve in great part at leas gj^ we find tliat it is in higher -^^ J^?^ chief part of those special arrange- for the aeration of this flmd ^ ^ ^^ forms in this class, ments are made, which J e meet Echinoderms live, It was formerly J^^th^ ^ ^ . g cer _ has free ^access to the ^ ca ^ of ^ and it is very doubtful S^t ccu^s ini; -The simplest provision for respiration in th. Sass is presented by the Sipunculida, which form a connecting link wxth the irticulated series. The viscera occupy but a small part o : the teneral cavity of the body, and this is filled with a corpusculated fluid K may /e seen (in «L of the smaller -mi-tr-sparent species^ perform a continual < cyclosis,' passing along the panetes oi the body trom ^Xchwtds as & as the proboscis, and then returmng , to ^e pos- terior extremity along the ^«**£*£Z* llL^tl^ this movement is sustained by ciliary action aM jm flfd\y hri"gin P g n « poslible to the circumambient water lie iin being o fenestrated at intervals, by the defrerency of the muscular SJy- only ^SSSStLtil • The Author «* tat feel ,^.^%£ZZ ^?S£d2?& WE should still doubt or eeen d»y 'ttf^^^St lee these animals ejecting minute teeala of Aetiate. Nothmg » more »« tta t ssed to fte attenipt to g-EK. t " P StpSfSa|t tLW fe «- speeies. (See Honard la "Ann. desSci. N^lfl^kurS'nas been denied, like that of the tentacular pores, + The existence «^^ l ^^ rticn l«r spe'cies examined. They are probably because ^^^^^Xn the tentacular pores ; but the following testimony seems most more frequently deficit than tw ^ ^ ^ .__„ The m ^ c ss explicit as to their e^*.^ in £ small stream from the perforated tubercles of eorni. and ^ allies ^°ften ejecte ^ ^ to h ht rf ^ legg tan £M£?* (5f JoUrf. «■« B^h Zoophytes," Yol. I., P ; 187.) ; { 296 OF RESPIRATION. tion to the surrounding medium. provision for the aeration of the (supposed) blood of these aahnil which conical, membranous tubes, passing between the pieces of the shelly framework, and projecting externally in little tufts; these (which were formerly, but erroneously supposed to be perforated at their extremities) are really branch^ for the aeration of the chylaqueous fluid which passes freely xnto them, and is moved to and fro by the action of cilia lining their interior, as m the tentacula of Actinia* Besides this, we find a system of vessels, which, though adapted to a special purpose in the economy of the animal, namely, the protrusion of the cirrhi, is probably subservient to respiration also. This consists of a central rin| around the mouth, which is furnished with numerous pedunculated vesicles, apparently contractile ; from this ring proceed five principal trunks along the five ambulacra; and these trunks seem to communicate with the double rows of vesicles (Fig. 37, e) from which the tubular cirrhi are sent iortn, that # serve as organs of prehension to these animals. The fluid contained in these tubes and vessels appears, from the observations of M. de Quatrefages and Dr. T. Williams, to be of the same nature with the fluid of the general cavity; and it cannot be questioned that, when pro- jected into the cirrhi, it must undergo an aerating change from its rela- There would seem to be no special . . . , . x pposed) blood of these animals, which, not being distributed to the extern al surface of the body, can only obtain oxygen either from the fluid introduced into the digestive cavity or from the chylaqueous fluid that bathes the trunks in which it circulates — 1 he respiration of the EcUnida seems to be carried-on upon a plan essential y the same ; except that the branchial ceeca, instead of bein^ dispersed oyer the general surface of the body, are collected into ten bundles, which pass through the flexible membrane surrounding the moutn. ihe apparatus for projecting the cirrhi is here very highly developed ; these appendages, when fully extended, being often several inches in length, so that the vesicles at their bases (Fig. 95?*) are required to be of considerable size-In the Holothurida, however, a far more 1 com- plicated set of provisions for respiration is found. In the first place, the muscular walls of the general cavity of the body are fenestrated, like those of the Sipuncuhda so that the chylaqueous fluid can be exposed to the aerating influence of the surrounding water through the skin alone Like the shell of the Echmida, again, they are perforated with cirrhi in connection with which the usual vascular apparatus is developed. Round the mouth is found a beautiful circle of tentacula (Fig. 40 6) which probably at once prehensile and branchial ; the parietes of these are fur- nished with true blood-vessels from the oral ring with which the laro-e con- tractile vesicle (Fig. 40,^) is connected, whilst their hollow axes are filled with chylaqueous fluid from the general cavity of the body. But we find in this group a true aquiferous or water-vascular system, which, though long considered anomalous, is now found to have its homolooues in an extensive group of the lower Articulata. This is the < respiratory tree,' which, in its higher grades of development extends from the cloaca into the visceral cavity in a beautiful arborescent form on each side of the body (Fig 40, r, r) thongh m the lower it is a simple undivided sac. tfoth the stem and branches of this oro-fm ^^ n ^ .«-!-:--* „_~_i .i * Sharpey, Op. cit., p. 616. WATER-VASCULAR SYSTEM. 297 longitudinal muscular fibres, which contract on being irritated, and which expel the water it contains, through its cloacal orifice, at tolerably regular intervals (such as once, twice, or three times in a minute), its re-intro- duction being apparently accomplished by ciliary action. The contraction of the muscular fibres of the external integument may probably assist m the expulsory action ; but it is not required, since the respiratory move- ment continues even after the sac has been laid open One of the respiratory .ratory mesenteric "system of vessels, will be brought into immediate relation with the aerating fluid introduced by it; the other branch lies m nearer proximity to the parietes of the body, and its respiratory action is pro- bably exercised rather upon the' fluid of the general cavity. • _ 2 85 It is among the Vermiform members of the Articulated series, that we find the < water-vascular' system acquiring its highest develop- ment But it will be desirable first to study it under the simpler form it presents in the Botifera, which have no rudiment of a sanguiferous system the chylaqueous fluid of the general cavity of the body being the medium alike for conveying nutriment to the solid tissues, and for effect- inc respiratory changes in them. On either side of the body of these animals there is usually found a long flexuous tube (Fig. 96, t, ^), which extends from a contractile vesicle (common to both) that opens into the cloaca, towards the anterior region of the body, where it frequently sub- divides into branches, one of which may arch over towards the opposite side, and inosculate with a corresponding branch from its tube. Attached to each of these tubes are a number of peculiar organs (usually from two to eight on each side) in which a trembling movement is seen, very like that of a flickering flame ; these appear to be pear-shaped sacs, attached by hollow stalks to the main tube, having a long cilium m the interior of each, attached by one extremity to the interior of the sac, and vibrating with a quick undulatory motion in its cavity ; and there can be no doubt that their purpose is to keep-up a continual movement m the contents of the aquiferous tubes, t— Similar lateral vessels, furnished internally with * On the respiration of the Echinodermata, see especially the Memoir of M de Quatre- fages < Sur la Cavite generale dn Corps des Invertebres ' ^"^J^^f' w %^' Zool Tom XIV. • and the less sound but more detailed Memoirs of Dr. 1. wmiams, ' Onke Mood proper and the Chylaqueous Fluid of Invertebrated Annuals,' in < Phdo . Transact " 1 8 52 • and < On the Mechanism of Aquatic Respmition, &c. , in Ann ol JN at. Sst " 2nd Ser Vols. 1 2, lS.-The whole of the vascular system connected with the cirrhi f regarded by Von lebolcL and others as belonging to his 'Water-vascular' or 'aquiferous syS but the Author does not see adequate reasou for so considering it, and thinks that it mudi more fairly ranks as a special apparatus exclusively belonging to this class. That it is at first developed, as the observations of Prof. Miiller have shown, by an inversion of the ex- ternal integument, and is for some time in free connection with the exterior, being only shut- off when its internal ramifications have been extensively developed is no argument agamst ?ne latte vSw- whilst it is much in favour of it, and against the determination of Von Sbold thlt I true water-vascular system exists, concurrently with the system of the S in tt Ho lothurida, -the group that presents the nearest approach to the Vermi- form MberL which this system is most characteristically displayed. 7 See IWey 'On LaciLnaria Socialist in "Transact, of Microsc ; Soc., New Series, Vol I Mr Huxley's description of these organs differs from that previously given by other observer; T who have supposed that these pyriform sacs had apertures of communication ^Z^P^tj of the body; but the Author has much confidence that Mr.Huxley's account is the correct one. ■i * H i*i* I I 8 ; 298 OF KESPIRATIOK. vibratile cilia, and often ramifying more minutely (especially in the head and anterior part of the body) are found in many of that group of Vermi- form animals, clothed over the whole surface of their bodies with cilia, to which the designation Turbellaria has of late been given. These vessels have been commonly regarded as sanguiferous; and it is certain that the fluid which they contain is sometimes coloured, like the (so- called) blood of Annelida;* but it is certain, also, that they have usually, probably always, external orifices, these being In Nais, instead of actually opening into the cloaca, one set of them comes into close relation to the rectum, the interior of which is richly ciliated; thus reminding us of the parallel distribution of the sometimes numerous. § seems Fig. 136. not improbable that the < water-vascular system of the lower Articulata (which are all aquatic) is the homologue of the tracheal system of the air- breathing Myriapods and Insects; and that, where it does not convey water directly introduced from without, the fluid which it contains is specially subservient to respiration, establishing a communication between the aerating surface and the tissues of the body generally. There is an almost complete absence among the Turbellarice, of any more special respiratory organs. The whole tegument of the body, being soft and clothed with cilia, is probably subservient to this function ; but there are occasionally to be observed about the head (as in Nemertes) particular groups of cilia longer and more closely set than the rest, reminding us of the 6 wheels 5 of the Rotifera. 286. This c aquiferous' system of vessels pre- sents its greatest complexity and most elevated character in the group of Entozoa ; whilst at the same time, there are certain types even of that class, in which it exists under its most simple and least doubtful aspect. It is only, in fact, by tracing it through its principal forms and grada- tions, that the real import of some parts of this system can be ascertained. In the Cestoid worms, we find four principal canals, two on each side (Fig. 136, a, b), running along the body at or near the margins of the segments, and connected together by transverse branches; these anasto- mose with one another freely in the head, those of the opposite sides being generally connected by an arched canal; whilst at the opposite ex- tremity they all terminate in a single contractile Segment of TcBniasoimm; sac opening externally. Besides these trunks (of SfffiSe? Vn^tuS ^hich the two larger, a, have been considered as eIch^dr k c C ova^ aS ^ al e^ a double alime ntary canal), there is a superficial teforiaee. C> ° VarJ ' ' g system of vessels more minutely distributed, which has been regarded as sanguiferous ; but these may be traced into connection with the preceding, and their parietes are furnished . * See the Memoir of M. de Quatrefages, v^*,~*~ .. , „ there is an alimentary canal ; both being replaced by the direct absorption of the nutritious juices from without, in a state ready for assimila- tion Yet it is probable that, as in the cases last cited, the 'water-vascular system contains some other fluid than pure water j and it may even serve as Prof. Van Beneden has suggested, for a urinary apparatus i he fluid contents of these vessels, whatever their nature maybe, are kept m fluid contents oi interior, and mrtlv bv the con- ciliary motion partly uy vw*j «~~ — — , * * » trar+Uitv of their walls; the former method seems to prevail in the tractility oi tneix , w™_Tti tnp Trpmratodp. Worms, we smaller trunks, the latter in the large. Worms, find a resular gradation from what is unquestionably a 'water-vas- cular' system homologous with that of the Cestoidea, to an apparatus ■t- \ w a-n^rnarhes the reputed sanguiferous system of the which ^^^^JtteJiing forms is that presented by the find Annelida. which we canal, termi- Distoma tereticolle ; in _ an elongated contractile nating posteriorly in an external orifice, siving off two contractile trunks, which pass along the two sides of the body with- out any change of dimension, and unite in an arch above the mouth. The fluid of these vessels is colourless, and contains some minute corpuscles. But besides these, there are a considerable number of smaller trunks, giving-off branches that form a net- work resembling that shown in Fig. 137 ; these trunks, however, have been distinctly found by M. Yan Beneden to discharge themselves into the two principal lateral canals, so that they must be considered as forming one system with them, although the fluid which they contain is coloured. This system of vessels, therefore, although usually described as sanguiferous, cannot be properly regarded in that light; and it is obvious, that even when (as happens the Echinorhynci, which are closely to the Trematoda) there are no pale, ciliated, non-contractile aquiferous vessels, terminating in an external orifice, but only red, non-ciliated, contractile vessels which cannot be traced into con- nection with the exterior, we must ab- Fig. 137. a in allied stain from regarding the latter as a true Anatomy ofFasciola hepatica (Distoma taj.il jj.v«« — © o . , . hepaticum) enlarged, showing .the ramifi- 'QQnm^ferOUS' System, Since tliey are ObVl- cationso fthe digestive cavity through the SangUlieiuus »y 9 CT .^V\ from thf> whole body of the animal, and the vascu- OUsly but an offset (SO tO Speak) irom tlie ™ e ^ k connected with, a median < aquiferons/ specially developed m this par- trunk; a , the mouth. 4.-1 ««« nf animals. — The nature 01 aetL 8 Z P sy 8 temTu the 1-ii™. (Fig. 52) 1- not yet been ,1 1 1 t 300 OF RESPIKATION. ground order also, what has been usually regarded as a sanguiferous system really belongs to the ' aquiferous' type. * 287. An arrangement of a different kind, but one that seems refer- nble to the < water-vascular' system of inferior Articulata, is found in the group Monoecious of Annelids. In the medicinal Leech, there is found on either side of the posterior part of the body a series of seventeen pairs of sacculi lying between the digestive coeca, and opening by narrow external orifices ; although these were long ago considered as respiratory organs yet they have been of late more generally regarded as organs of secretion t their homological character, however, as a < water-vascular' system, appears to be demonstrated by the existence of intermediate forms, which connect it with the aqiiiferous system of Entozoa. Thus in Branchioldella there are but two pairs of external orifices, one at the anterior and the other at the posterior extremity of the middle third of the body; each of these leads to a trunk, which, after dilating into an ampulla, gives off several tortuous canals, the lining of which is ciliated. In the Earthworm, again, there is found in each segment, and on either side of the digestive' tube, an enteroid vessel which returns upon itself, and which is lined with cilia; and in other Lumbricidse, these vessels have cajcal termi- nations in the general cavity of the body, furnished like the water- vessels of Rotifera (§285), with long cilia. }— Nothing similar to this has been found among the branchiferous Annelida ; and it would seem as if the water-vascular system were superseded in them by the apparatus provided for aquatic respiration, which will be hereafter described (§ 292). The close resemblance which seems to exist between the multiple sacculi of the Leech, and the air-sacs of the lower Myriapoda (§ 301), strengthens the reasons already advanced (§ 285), for regarding the ' water-vascular' system as the real homologue of the tracheal system of Myriapods and Insects. And if the suggestion already thrown out (S 219), respect- ing the real nature of the supposed sanguiferous system of Annelida, should prove well-founded, this also would find its parallel in the closed tracheal apparatus of certain Insect-larva* (§ 305), which is connected like it, with a branchial apparatus; the difference between the two bein^ that the latter contains and distributes air, whilst the former is filled with an aeriferous fluid, which can scarcely be called blood the distri- bution of nutritive materials being accomplished in both cases bv the fluid of the ' general cavity of the body.' 288. Branchial Respiration. — In by far the larger proportion of * See on this most important subject, the treatise on "Les Vers Cestoides " bv Prof Van Beneden (Bruxelles, 1850) ; and his Note ' Sur l'appareil circulatoire des Trema- todes,' m "Ann. des Sci. Nat.," 3 e Ser., ZooL, Tom. XVII. Also Siebold, ' Ueber den Generation's wechsel der Cestoden,' in "Siebold and Kolliker's Zeitsch'rift " 1850' Wagener, in "Miiller's Archiv.," 1851 ; and "Brit, and For. Med. Chir Kev "Vol X ' pp 324—6.— The Author is informed by his friend Mr. Huxley, that he has recently detected m the A scans (a Nematoid Entozoon) of the Plaice, two non-ciliated lateral con- tractile vessels, extending the whole length of the body, and united over the cesophagus bv a transverse branch which appeared to open externally. + See Quatrefages, in "Ann. des Sci. Nat.," 3 e Ser., ZooL, Tom. XIV., p 297 1 These terminations have been affirmed by good observers to be sometimes open. See x^eydig on Branchioldella, "Zeitschrift fur Wissen. Zool.," Band III., and Gegenbaur on t>ne harthworm, Ibid., Band IV. Mr. Huxley has also seen them in Nais. BRANCHIAL RESPIRATION IN TUNICATA 301 Molluscous dom the classes of Annelida and Crustacea among the Articulated, with Fishes and Batrachia (at least in their early or larval condition) among the Vertebrated, we find the circulating fluid transmitted for aeration to external appendages, which are disposed in the various forms of leaflets, fringes tufts, &c, and the water in contact with which is continually re- newed,' sometimes by the action of the cilia that clothe their surfaces, some- times by a muscular apparatus specially adapted for the purpose. These branchiae however, are not always apparent externally ; being frequently enclosed 'in some extension of the integument, which either simply covers them in or which forms a special chamber for their reception, with orifices of entrance and of exit for the water that is conveyed over the respiratory surface. Molluscous piration is most characteristically developed, we shall first examine the principal forms of the branchial apparatus which it presents j and shall then proceed to notice its chief peculiarities m Articulated and m Ver- tebrated animals. . . , „ 289 The lowest division of this series, however,— namely, the group of Bryozoa —can scarcely be said to possess any special respiratory apparatus, the degradation of their circulating system extending itself also to this; still the aeration of the nutritious fluid that occupies their visceral cavity seems to be provided-for, by its transmission along channels in the interior of their ciliated tentacula (Fig. 49, D, b, b) ; and it has also been observed that the wide pharynx is sometimes dilated with water, which is expelled again without passing into the stomach, as if it had been taken-m for the aeration of the fluids of the body. In the class of Tunicata, of which the Bryozoa may be regarded as a degraded form, we find (save m a few aberrant genera) a highly-specialized apparatus for the performance of the respiratory function. This apparatus is always connected with the entrance to the alimentary canal ; and the currents of water set m motion by the cilia that cover it, serve (like those impelled by the ciliated crown of the Bryozoa) the double purpose of bringing fresh supplies of food to the digestive cavity, and of water to the aerating surface. Two types ot conformation, that are at first sight very different, present themselves m the respiratory apparatus of this class ; the most simple being that of the SalpidZ, the most elaborate that of the ordinary Asc tdwns. J* **?«*; pid* we' find the large cavity that occupies the S^f^^^Z of the body, to be traversed by a riband-like fold of its lining membrane (FiTm, a, d), which stretches obliquely from its anterior attachment a little behind the oral orifice (a), to its posterior attachment to the visceral nucleus between the oesophageal orifice and the rectum ; thus partially dividing the cavity into two parts, an anterior or pharyngeal, and a pos- terior or cloacal. The anterior is really the dilated pharynx, which, in an early stage of the development of these «^.^^^<^ Brvozoa) from the oral orifice to the entrance of the oesophagus , a con- commences m the embryo as a simple depression ux * See Dr. A. Farre < On the CiloWuate Polypi,' * " ^ ilos ' Transact.," 1837, p. 407. 302 OF RESPIRATION, extends inwards so as to meet the pharyngeal sac, with which it comes to communicate by a large aperture on each side ; and this gradually widens, until the whole forms one respiratory sac, of which the alimentary canal (both of whose orifices are received into it) seems but a diverticulum. The branchial lamella, formed by the united lining membranes of the pharyngeal and cloacal cavities, is traversed by an extension of the great ' sinus-system, 5 which passes-in between its two layers ; in some species, only a single grand sinus runs through it ; but in others there is a com- plex system of vascular ramifications, extending from this sinus over its surface, as shown in Fig. 122. It is beset with cilia on either side of its free edge ; and by the action of these a current of water is continually drawn-in through the oral orifice, and propelled backwards, part of it entering the oesophagus, but another part at once passing into the cloacal portion of the cavity, to be expelled through the anal orifice or vent (&). Although this organ is undoubtedly the special instru- ment of respiration, yet it can scarcely be questioned that the whole lining membrane of the cavity is also subservient to the same action, especially where this is minutely vascular throughout, as shown in Fig. 122, B. Ciliary action is not the only — perhaps not the prin- cipal — means of renewing the water required for respiratory purposes in the Salpidse; for they execute rhythmical movements of alternate Fig. 138. B \ N \ A, Group of Perophora (enlarged), growing from a common stalk : — B, single Perophora; a, test ; b, inner sac ; c, branchial sac, attached to the inner sac along the line c' c' ; e,e, finger-like processes projecting inwards ;y, cavity between test and internal coat ; f', anal orifice or funnel ; g, oral orifice ; g' t oral tentacula ; h, downward stream of food ; h'l oesophagus; i, stomach; Jc, vent; I, ovary (?) ; n, vessels connecting the circulation in the body with that in the stalk. contraction and dilatation, the former being accomplished by the instru- mentality of their muscular bands, the latter by the elasticity of the ' test 5 .. BESPIHATION IN TUNICATA. 303 Fig. 139. when the muscular action ceases ; and in this manner the water received into the cavity, being prevented from returning through the oral orifice by a bilabiate valve which closes it, is ejected through the anal, with a force that drives the animal in the opposite direction. — In all the tribes which make-up the group of Ascidians, on the other hand, the dilated pharynx (Fi<*. 138,B,c) is completely embraced by the inflected cloacal sac, save alono* the dorsal line, where the great thoracic sinus intervenes ; and the communication between them is established by a number of vertical slits arranged in regular rows, and fringed with cilia very closely set together (Fi<*. 139). This peculiar conformation, like that of the Salpidse, is only attained as embryonic development advances ; for the cloacal sac, which commences as a superficial depression in the position of the anal orifice, is gradually inflected inwards, until it almost entirely surrounds the pharynx ; and in the pharyngeal wall thus formed by the coalescence of the two contiguous membranes, one aperture is formed after another, until the entire series is completed. The spaces between the rows of slits are occupied by thickened trans- verse bars; and these contain extensions of the sinus-system, along which the circulating fluid passes between the great sinus which runs vertically along the dorsal margin and the other which passes along the ventral margin. The spaces between the slits of the same row are not thus thickened, but they seem to contain extensions of the sinus-system, as globules of blood may be seen to move in them. From the transverse bars, digitiform processes are occasionally sent into the cavity ; and the oral orifice is usually fringed with a set of tentacula projecting inwards. The use of these would appear to be, to receive impressions from par- ticles drawn-in by the respiratory current, whose presence is unsuitable j these impressions serving to excite a contraction of the muscular sac, whereby a gush of water is expelled through one or both orifices, an action analogous to that of coughing in the higher animals, such muscular movements are concerned, however, in sustaining the ordinary respiratory current ; this being entirely kept-up by the action of the cilia, here so abundantly distributed. The greater part of the water thus drawn-in, is at once transmitted through the slits in the branchial sac, into the cloaca, whence it is ejected by the vent; the solid particles fit for food, however, are retained, and are gradually carried- down to the entrance of the oesophagus : and the small stream of water which passes with them into the alimentary canal, is discharged through the intestine into the cloaca, so as to pass-out by the vent. — Notwith- standing the apparent difference between these two plans, they are really reducible to a common type ; as is shown alike by the history of their development and by the occurrence of intermediate forms, such as Doliolum and Pyrosoma, which establish the transition from one to the The curious Appendicularia permanently resembles, both in its Portion of branchial sac of JPerophora, highly magnified, showing the slits lined with cilia, and the currents of blood flow- ing between them. No other. ipiratory ■ ii *HM • 304 OF RESPIRATION. Fig. 140. the larvae of the Ascidians. For it possesses no cloacal cavity, the intes- tine opening directly on the external surface; the dilated pharynx has therefore no internal communication, save with the alimentary canal ; and the water which is taken into it for respiratory purposes, and is not re- quired to pass through the intestinal tube, is ejected again by the oral orifice. Thus, whilst possessing the peculiar conformation of a larval Asci- dian (being unattached, and moving freely through the water by a long caudal appendage, like an Amaroucium in its early state, Fig. 246), this curious being shows but a very slight depar- ture from the Bryozoic type. For if the tentacular circle of a Bryozoon were to become rudimentary, if the general cavity of its body were to be contracted, by the partial adhesion of its opposite walls, into a sinus-system, and if one por- tion of this should acquire a contractile coat, we should have all the essential parts of the digestive, circulatory and respiratory apparatuses, nearly in the relations which they actually bear to one another in Appendicularia.* 290. Passing-on now to the Bivalve Mollusks, we find that the sions shells of the mantle which expan- line the Interior view of Pholas crispata, the mantle being laid open along the ventral margin :— a. incurrent siphon, and 5, excurrent siphon, having whalebone probes passed through them into the chambers with which they respectively communicate ; c, branchial laminae ; d, anal chamber laid open, showing four rows ot orifices leading to tubes between branchial lamina? ; e, e, labial tentacles ;/, accumulation of particles of indigo, propelled along the edges of the gills; g } oral orifice ; h, toot. of the Brachiopoda, and over which the blood is freely distributed, constitute their principal organ of re- spiration ; the surrounding water being freely admitted to the space between the valves, and being probably kept in motion by the agency of the cilia that clothe the ' arms.' In those species in which the shell is perforated by csecal extensions proceeding from the great sinus-system within each valve to its ex- ternal surface (§236 note), it is probable that these, like the papillse of the Nudibranchiate Gasteropods (§ 291), may serve as an additional means for aerating the blood. As the blood which circulates among the viscera and in the ciliated arms must be brought into aerating relation to the water that is in contact with every part of the * See the Monograph of Prof. Milne -Edwards, " Sur les Ascidies Composees" (1840) ; the Memoirs of Mr. Huxley, on the ' Anatomy of Salpa and Pyrosoma,' and on ' Doliolum and Appendicularia, ' in "Philos. Transact.," 1851; and the Notice by the same excellent and philosophical observer in the " Report of the British Association," 1852.— A very in- genious idea of the Homology of the Organs of the Tunicata and Polyzoa (Bryozoa) has been put forth by Prof. Allman in the " Transact, of the Roy. Irish Acad.," Vol. XXII.; but the Author agrees with Mr. Huxley in considering this idea to be negatived by the pheno- mena of development, as observed by Krohn, "Miiller's Archiv.," 1852. RESPIRATION IN LAMELLIBRANOHIATA. 305 Fig. 141. * thin integument, there seems the less reason for any peculiar specialization of the respiratory apparatus. The Lingula (Fig. 82) presents in some degree a transition from this low type, towards the higher condition of the Lamel- libranchiate bivalves ; its mantle being thrown into plaits or folds, which prefigure their lamellated branchhe * — In the Lameltibranchiate Bivalves, we find the internal surface of the expansions of the mantle that line the valves, to be doubled (as it were) into four riband-like folds (Fig. 140, c), which are very minutely supplied with blood-vessels (Fig. 123, f, f), and which are obviously the special organs of aeration, though the general surface of the mantle still doubtless participates in that function. Of the four branchial lamellae, two are attached to each lobe of the mantle ; and each of them con- sists of two folds of its mem- brane (as will be seen in Fig. 141, c), which are continuous at its edge (c) and are attached at certain points of their contigu- ous surfaces (k), but are sepa- rated at others by intervening channels (g, g)^ the interval be- tween them being widest at the base. Only one of these laminse is usually attached (f), the borders (e e,) of the others being free, so that a probe may be passed between them into the interior spaces (g, g) between the layers. The structure of these layers varies considerably in the different families of the class; for in some instances they are continuous membranes, perforated (like the branchial sac of the Ascidians) with slits arranged in rows and fringed with cilia, the intervals between which are occupied by blood- channels; whilst in other in- stances they seem made-up of ciliated bars containing blood- channels, but having few points of union with each other. In ^ either case, however, the effect is the same. The water is propelled over the surface of the branchial lamellae in a constant stream, by the action of the cilia that border their* fissures, whilst the blood is made to traverse the spaces intervening between these fissures, and is thus effectually aerated. The water that has performed this office, passes through the fissures into * The term P alliobranchiata has been proposed by Prof. Owen as an appropriate desig- nation for this group, being indicative of the performance of the respiratory function by the general surface of the mantle. Branchial apparatus of Lamellibanchiate MoUusk :— A portion of branchial lamina* highly magnified show- in/ orifices in the meshes of the vascular net-work, fringed with cilia :— b, section of gill-plate, showing two of the tubes formed by the adhesion of its laminae: c ideal section of the two branchiae ot one side ; ec, 'free layer of each gill; fc, its attached layer; q a, spaces between the layers, communicating with the anal chamber; h, interval between the contiguous lamellae ; k Jc, bars connecting the two folds of the same lamella. * X t 306 OF RESPIRATION. the spaces (g, g) between the branchial lamellae, and is discharged in a stream from the extremity which is nearest the anal outlet. It is only in a comparatively small number of Lamellibranchiata, that the two lobes of the mantle are free (as in the Oyster) along the entire margin of the valves, so that the water can enter at any point ; for they are generally found to be more or less united, so that the gills become enclosed within a branchial cavity. There is always a provision, however, for the free access of water from without, by means of two apertures (Fig. 140, a, b), one of which serves for its entrance, and the other for its ej ection ; and in certain species which burrow deeply in sand, mud, wood, &c, these apertures are furnished with long tubes or siphons reaching to the en- trance of the burrow, which are themselves lined by cilia. In the Pholas (Fig. 140) and its allies, among which the closure of the mantle-lobes is carried to its fullest extent, the cavity is divided into a branchial and an anal portion, which have no other communication the one with the other than through the fissures in the branchial lamellae. The two laminae of each gill are in such close adhesion to each other, that the intra- branchial spaces (Fig. Ul,c,g,g) are reduced to a set of parallel tubes (as shown at b c), which open into the anal chamber. Thus the water that enters by the branchial or incurrent siphon (Fig. 140, a), after finding its way through the slits in the branchial lamellae (shown in Fig. 141, b, and more highly magnified at a), into the intrabranchial tubes, passes at once into thermal chamber, and is discharged by the anal siphon (Fig. 140, 6). In its course over the branchiae, however, it is deprived not merely of its oxygen, but also of whatever alimentary particles it may contain ; and these are collected, by a very peculiar adaptation of the ciliary mechanism, on the free edges of each lamina (/), along which they gradually travel to the orifice of the oesophagus (g) ; as has been shown experimentally by diffusing particles of indigo through the water thus introduced.* As in the Tunicata, the presence of an obnoxious particle will cause its ej ection with a j et of water through the branchial orifice; the branchial cavity being forcibly pressed-on by the valves of the shell, which are drawn together by the adductor muscles. 291. The class of Gasteropoda is remarkable as being the only one in the entire series of Mol- lusca, that contains animals adapted for atmo- spheric respiration. The ' pulmonated' Gastero- pods, however, are comparatively few in number ; by far the larger proportion of this group being- provided with branchiae, in which their blood is aerated by the contact of water. These bran- chiae in many orders form tufts of an arbo- rescent character (Fig. 142), which may be disposed upon various parts of and Fig. 1A2 One of the arborescent pro- cesses, forming the gills of Doris Johnstoni, separated and en- larged. * See especially, upon this subject, Prof. Sharpey's Art. ' Cilia,' in " Cyclop, of Anat. ar Physiol " Vol. L, p. 622 ; and Messrs. Alder and Hancock, in " Ann. of Nat. Hist., 2ndSer'.' Vol. VIII. , 1851, p. 370. — Mr. Clark, in various numbers of the same journal, has endeavoured 'to prove that water is ordinarily taken-in by the ' pedal gape,' and is dis- charged by the branchial as well as by the anal siphon. But the Author considers that there is ample evidence that such is not the ordinary course of the current. ~ — ** BESPIKATION IN GASTEROPODA AND CEPHALOPODA. 307 Fig. 143. Boris Johnstoni, showing the tuft of external gills. the surface of the body, or may be collected into a single cluster (Fig. 143). There can be no doubt that, in the NudibrancMate Mollusks, the gene- ral surface of the body takes an important share m the aeration of the blood ; and it appears that in some instances it must constitute the princi- pal, or even the sole instru- ment for the performance of this function, since there is little or no trace of any special respiratory apparatus. We have seen that the former is the case in regard to Doris ; it being only ^e bloody ^f kid and ovaria , that is sent to has circulated throng * g» ™ ''£L&* is transmitted for aeration the branchial circle^ whilst t ^ ^ ^ ^ ^ ^ ^ to the skm. In Jboto <* g ; instnme nt of aeration, no more papilke seems to ™^»™^ J discovera ble j and the whole of special provision ^ for ^JW^ ^ gurface of tlie papilke tham- ZZ%T^^' In some members of this Wy,thesur- ?acl is further extended by the peculiar conformation of the papilla everv one of which is furnished with a sort of membranous frill across an efferent vessel that runs along the tree mar & i , meeting veys the blood back to a great trunk-vem .that ^^^^ of all these papil ary vessels ^^^ £ the sliel i . and even Gasteropods, the gills are more or less cov ered oy where this is but ^^^"^pf^^a, whii is t/e devoted to their protection. X he oruei rece ives its name highest and most *™™^^^J^2rt of its gills from the peculiar pec ^ated or comb n*e y ^ ^ (Fig. 144) which are lodged Y^^X^^ is admitted by * mantle behind the head • and mt £ this c avi ty ^ ^.^ q£ ^ special channel or siphon.— The ^f>J*« t d b distribution to special respiratory organ ; ^^Z^luZeof winch is minutely the general surface of the mantle, the inner ^su aratus pre . vascular.-The highest development^of g the bra n PP ^ sented by the Mollusca, xs ^J^^^eeially in the Dibranchiate these animals, the gills are very ^large, espec y ^^ order (Fig. 125, o £ -^To Xch water is admitted through 1 the fokimg-over of th J e ^ ^ ^ ^